Post-deposition encapsulation of nanostructures: compositions, devices and systems incorporating same

ABSTRACT

Ligand compositions for use in preparing discrete coated nanostructures are provided, as well as the coated nanostructures themselves and devices incorporating same. Methods for post-deposition shell formation on a nanostructure and for reversibly modifying nanostructures are also provided. The ligands and coated nanostructures of the present invention are particularly useful for close packed nanostructure compositions, which can have improved quantum confinement and/or reduced cross-talk between nanostructures.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a non-provisional utility patent applicationclaiming priority to and benefit of the following prior provisionalpatent applications: U.S. Ser. No. 60/578,236, filed Jun. 8, 2004,entitled “POST-DEPOSITION ENCAPSULATION OF NANOCRYSTALS: COMPOSITIONS,DEVICES AND SYSTEMS INCORPORATING SAME” by Jeffery A. Whiteford et al.,and U.S. Ser. No. 60/632,570, filed Nov. 30, 2004, entitled“POST-DEPOSITION ENCAPSULATION OF NANOSTRUCTURES: COMPOSITIONS, DEVICESAND SYSTEMS INCORPORATING SAME” by Jeffery A. Whiteford et al., each ofwhich is incorporated herein by reference in its entirety for allpurposes.

FIELD OF THE INVENTION

The invention relates primarily to the field of nanotechnology. Morespecifically, the invention pertains to compositions, devices andmethods involving discrete coated nanostructures.

BACKGROUND OF THE INVENTION

Individual nanostructures, as well as those embedded in other materialsto form nanocomposite materials, have many promising applications,including applications that make use of their optical and electronicproperties. One particularly useful application would be in the area ofnanocomposite based memory, where the nanostructures allow for highdensity charge storage.

Of the synthetic approaches available for preparing nanostructures,top-down patterned approaches such as chemical vapor deposition (CVD) ormolecular beam epitaxy (MBE) are commonly used to generate core andcore:shell nanostructures. These methods typically yield large and/ordisordered and/or low density packing nanoparticles, and require highcost (high temperature, high vacuum) processing steps. Solution basedsyntheses can also be used to synthesize semiconductor nanocrystals(either cores or core/shells) which are more readily compatible withsolution based deposition methods such as spin coating or otherevaporation methods. For example, nanostructures comprising CdSe cores(or crystalline cores) with a shell of ZnS can be prepared by solutiondeposition techniques (see, for example, Murray et al (1993) “Synthesisand characterization of nearly monodisperse CdE (E=S, Se, Te)semiconductor nanocrystals” J. Am. Chem. Soc. 115: 8706–8715). However,nanostructures generated by these and other standard core-shellsynthetic techniques typically do not have a thick enough shell toconfine a charge in the core to enough degree to prevent chargediffusion to other nanostructures placed within a few nanometers of thefirst nanostructure.

Alternatively, nanostructure synthesis by a chemical self-organizingapproach typically produces the most well-controlled morphology andcrystal size, but these synthetic protocols generate nanostructureshaving associated therewith additional organic and/or surfactantcompounds. While useful for enhancing solubility and facilitatingmanipulation of the nanostructures during synthesis, the organiccontaminants are avidly associated with the nanostructure surface, thusinhibiting further manipulation and/or integration of the newlysynthesized nanostructure into devices and end applications.

Even if these CdSe:ZnS constructs could be prepared having diametersallowing for high density packing (e.g., about 1×10¹²/cm² or greater),the ZnS shell would not provide enough quantum confinement for efficientuse of the nanostructures in microelectronic and photonic devices,including, but not limited to, memory or charge storage devices.

Accordingly, there exists a need in the art for discrete coatednanostructures that can be easily integrated into various manufacturingprocesses without further processing. Preferably, the coatednanostructures can be closely packed while maintaining greater quantumconfinement than standard CdSe/ZnS core:shell structures. The presentinvention meets these and other needs by providing discrete coatednanostructures, ligands for coating discrete nanostructures, devicesincorporating the coated nanostructures, and methods for preparing thecoated nanostructures. A complete understanding of the invention will beobtained upon review of the following.

SUMMARY OF THE INVENTION

One general class of embodiments provides a discrete coatednanostructure. The discrete coated nanostructure includes an individualnanostructure having a first surface, and a first coating associatedwith the first surface of the individual nanostructure. The firstcoating has a first optical, electrical, physical or structuralproperty, and is capable of being converted to a second coating havingone or more of a different optical, electrical, physical or structuralproperty than the first coating. In some embodiments, the first coatingencapsulates the nanostructure; in other embodiments, the first coatingcovers a portion of the nanostructure (for example, the portion of thenanostructure not associated with the surface of a substrate). In oneembodiment, the electrical property of the second coating is adielectric property; exemplary second coatings for this embodimentinclude silicon oxide, boron oxide, and combinations thereof.

Nanostructures that can be used to prepare the discretely coatedcomposition of the present invention include, but are not limited to,nanocrystals, nanodots, nanowires, nanorods, nanotubes, variousnanoparticles, including, e.g., metal, semiconductor, or insulatornanoparticles, metal nanoparticles such as palladium, gold, platinum,silver, titanium, iridium, cobalt, tin, zinc, nickel, iron or ferritenanoparticles or alloys of these, amorphous, crystalline, andpolycrystalline inorganic or organic nanoparticles, and polymericnanoparticles, such as those typically used in combinatorial chemicalsynthesis processes, e.g., like those available from Bangs Laboratories(Fishers, Ind.), nanotetrapods, nanotripods, nanobipods, branchednanostructures, branched nanocrystals, and branched tetrapods. In apreferred embodiment, the nanostructure comprises a spherical, nearlyspherical, and/or isotropic nanoparticle such as a nanodot and/or aquantum dot. Preferably, the coated nanostructure has at least onedimension (for example, a diameter of the coated nanostructure) that isless than about 10 nm, and optionally less than about 8 nm, 5 nm, or 4nm. In some embodiments of the present invention, the diameter of thecoated nanostructure is between about 2 nm and about 6 nm, e.g., between2–4 nm.

A number of ligand compositions can be employed as coatings for thenanostructure. In one class of embodiments, the second coating comprisesan oxide (e.g., SiO₂). In some embodiments, the first coating has afirst component comprising a silicon oxide cage complex and a secondcomponent comprising one or more nanostructure binding moieties.Exemplary nanostructure binding moieties include either the protonatedor deprotonated forms of phosphonate, phosphinate, carboxylate,sulfonate, sulfinate, amine, alcohol, amide, and/or thiol moieties.Preferred nanostructure binding moieties include ester moieties ofphosphonate, phosphinate, carboxylate, sulfonate, and sulfinate.Typically, the nanostructure binding moieties are independently coupledto the silicon oxide cage complex, e.g., via an oxygen or silicon atomof the cage.

In certain embodiments, the coated nanostructure includes asilsesquioxane composition as the first coating. The silsesquioxane canbe either a closed cage structure or a partially open cage structure.Optionally, the silicon oxide cage complex (e.g., the silsesquioxane) isderivatized with one or more boron, methyl, ethyl, branched or straightchain alkanes or alkenes with 3 to 22 (or more) carbon atoms, isopropyl,isobutyl, phenyl, cyclopentyl, cyclohexyl, cycloheptyl, isooctyl,norbornyl, and/or trimethylsilyl groups, electron withdrawing groups,electron donating groups, or a combination thereof. In an alternateembodiment, discrete silicates are employed in the first coatingcomposition. One discrete silicate which can be used as first coatingsis phosphosilicate. Upon curing, the silicon oxide cage complex firstcoating is typically converted to a second rigid coating comprising asilicon oxide (e.g., SiO₂).

The coatings employed in the compositions of the present inventiontypically exhibit a first property in their initial (i.e.,pre-conversion or pre-cured) state, and a second, differing property inthe second, post-conversion or post-curing state. For examples involvingcoatings having differing electrical properties upon conversion orcuring, the first electrical property could include conductivity whilethe second electric property is nonconductivity (or vice versa).Likewise, the material in the first state may be an electron conductoror a neutral material, while the material in the second state may be ahole conductor. Alternatively, for embodiments relating to opticalproperties, the first and second optical properties could be opacity andtransparency, e.g. to visible light. Alternatively, the first opticalproperty could include light absorption (or transmission or emission) ata first wavelength, while the second optical property comprises lightabsorption (or transmission or emission) at a second wavelength.Alternatively, for embodiments relating to structural properties, thematerial in the first state could be a flexible molecule, while thesecond state could comprise a rigid (porous or solid) shell. In oneclass of embodiments, the first physical property comprises solubility,e.g., in a selected solvent, while the second electrical propertycomprises nonconductivity. Conversion of the coating can beaccomplished, e.g., by application of heat and/or radiation.

The present invention also provides an array comprising a plurality ofdiscrete coated nanostructures. In a preferred embodiment, the membernanostructures are present at a density greater than about 1×10¹⁰/cm²,greater than about 1×10¹¹/cm², and more preferably at greater than about1×10¹²/cm² or even greater than about 1×10¹³/cm². Optionally, the membernanostructures are associated with a surface of a substrate, such as asilicon wafer. In some embodiments, the member nanostructures areencapsulated prior to association with the substrate surface, while inother embodiments, a first portion of a member nanostructure isassociated with the substrate, and a second portion of the membernanostructure is associated with the first coating or the secondcoating. Optionally, the surface of the substrate includes asurface-binding ligand coupled to a second nanostructure binding moiety,e.g., for association with a portion of the nanostructure surface. Forexample, in the case of a silicon wafer, a silane moiety would functionas the binding ligand on the substrate or surface.

Devices including a plurality of discrete coated nanostructures formanother feature of the invention. Exemplary devices that can incorporatethe discrete coated nanostructures of the invention include, but are notlimited to, a charge storage device, a memory device (e.g., a flashmemory device), and a photovoltaic device.

In another aspect, the present invention provides a coatednanostructure-containing composition having a plurality ofnanostructures and a coating separating each member nanostructure. Thecoating includes a plurality of nanostructure binding moieties attachedto a surface of the member nanostructure; after association of thenanostructure binding moieties with the surface of the membernanostructure, the coating can be converted to the second coating (e.g.,an insulating shell; the first coating is optionally also insulating).Optionally, the second coating or “shell” is an inflexible structurethat provides a spacing (e.g., a selected or defined distance, or rigidspacing) between adjacent member nanostructures. For example, dependingupon the coating employed, the diameter of a given coated nanostructure(or the distance from center to center between adjacent nanostructuresin a packed array) can range, e.g., between about 1 and about 100 nm, oroptionally between about 1 nm and about 50 nm. In preferred aspects, ahigher packing density is desired, and thus a distance betweennanostructures optionally ranges from about 1 nm to about 10 nm, about 3nm to about 10 nm, and more preferably, between about 2 nm and about 6nm, e.g., between about 3 and about 5 nm or about 2 nm and about 4 nm.In certain aspects for which a thickness that provides acceptableinsulation or coating thickness while preserving a high packing densityis preferred, the diameter of the coated nanostructure falls within arange of from about 2 nm to about 6 nm, or optionally about 3.5 nm (orless).

In some embodiments, the insulating shell reduces or prevents (e.g.,lateral) charge diffusion or transmission between adjacent or proximalmember nanostructures, or between a nanostructure and another adjacentor proximal material or substrate. Alternatively, the shell may reduceor prevent other types of transmission, such as light or heat. In oneclass of embodiments, the insulating shell reduces the rate of chargediffusion between member nanostructures, whereby the average time for anelectron to hop from one member nanostructure to another is greater thana predetermined length of time (e.g., greater than 1 millisecond, 1second, 1 minute, 1 hour, 1 day, 1 month, or even 1 year or more).

The member nanostructures can be associated with a surface of asubstrate. In one class of embodiments, the substrate comprises asilicon substrate. The silicon substrate can comprise a functionalizedor oxidized silicon substrate. In one class of embodiments, the siliconsubstrate further comprises a silane ligand coupled to a secondnanostructure binding moiety. The plurality of nanostructure bindingmoieties and the second nanostructure binding moiety can be similarchemical moieties, or they can be disparate chemical moieties.

Nanostructure binding moieties that can be employed in the compositionsof the present invention include, but are not limited to, one or morephosphonate ester, phosphonic acid, carboxylic acid or ester, amine,phosphine, phosphine oxide, sulfonate, sulfinate, alcohol, epoxide,amide or thiol moieties. The coating used to form the insulating shellcan be an organic, an inorganic, or a hybrid organic/inorganiccomposition. In some embodiments of the present invention, thenanostructure-binding coating comprises a silicon oxide cage complex,such as one or more silsesquioxanes or discrete silicates.

Essentially all of the features described for the embodiments aboveapply to these embodiments as well, as relevant; for example, withrespect to type of nanostructures, density of member nanostructures,association with a substrate, inclusion in devices, and/or the like. Thecomposition optionally includes a topcoat composition, e.g., onecomprising the same material as the coating or the insulating shell.

In a further embodiment, the present invention also provides a pluralityof discrete nanostructures encompassed with rigid SiO₂ shells, wherein adiameter of a member nanostructure:shell construct (i.e., a membernanostructure with its shell) is less than about 10 nm (or optionallyless than about 8 nm, less than about 6 nm, less than about 4 nm, orless than about 3.5 nm), and/or wherein the member nanostructures arepresent at a density greater than 1×10¹⁰/cm², or optionally greater thanabout 1×10¹¹/cm², about 1×10¹²/cm², or even equal to or greater thanabout 1×10¹³/cm². The member nanostructures are optionally arranged inan array, e.g., an ordered or disordered array. Essentially all of thefeatures described for the embodiments above apply to these embodimentsas well, as relevant; for example, with respect to type ofnanostructures, association with a substrate, inclusion in devices,topcoats, and/or the like.

In one class of embodiments in which the plurality of discretenanostructures are encompassed with rigid SiO₂ shells, a diameter of amember nanostructure with its shell is less than about 6 nm, and themember nanostructures are present at a density greater than 1 ×10¹²/cm²,the member nanostructures are associated with a surface of a substrate.The plurality of nanostructures can further comprise a top coating. Thetop coating optionally comprises a silicon oxide top coating.

The present invention also provides devices, systems, compositions,films, and the like having therein a plurality of discrete coatednanostructures. One exemplary device that could be used with thediscrete coated nanostructures of the present invention is a memorydevice, e.g., a flash memory device. In a preferred embodiment, theflash memory device includes a plurality of discrete nanostructuresencompassed with rigid SiO₂ shells, wherein a diameter of a membernanostructure is less than about 6 nm, and wherein the membernanostructures are present at a density greater than about 1×10¹⁰/cm²,or more preferably, densities greater than about 1×10¹²/cm². Otherexemplary devices include charge storage devices and photovoltaicdevices.

In a further aspect, the present invention provides methods forpost-deposition shell formation on a nanostructure. The methods includethe steps of providing one or more nanostructures having a ligandcomposition associated with a first surface, which ligand composition iscapable of being converted to a rigid shell, and converting or curingthe ligand composition and generating the rigid shell on the firstsurface of the nanostructure, thereby forming the shell after depositionof the ligand composition. The ligand composition can be, e.g., any ofthose described herein.

In one class of embodiments, the ligand composition comprises aplurality of nanostructure binding moieties coupled to a silicon oxidecage complex. The silicon oxide cage complex can comprise asilsesquioxane composition. In one class of embodiments, the rigid shellcomprises an electrically conductive composition. In one class ofembodiments, the rigid shell comprises an electrically insulatingcomposition. In one class of embodiments, the rigid shell comprises anoptically transparent composition.

The nanostructures can be provided by synthesizing one or morenanowires, nanorods, nanotubes, branched nanostructures, branchednanocrystals, nanotetrapods, nanotripods, nanobipods, nanocrystals,nanodots, quantum dots, nanoparticles, or branched tetrapods (or acombination thereof) by any of a number of techniques known in the art.For some embodiments, providing the one or more nanostructures involvesproviding semiconductor nanocrystals or metal nanocrystals having atleast one dimension of less than 10 nm, less than about 5 nm, or between2–4 nm or smaller.

In one class of embodiments, the nanostructures having a ligandcomposition associated with a first surface are provided by providingone or more nanostructures having one or more surfactants associatedwith the first surface and exchanging the surfactants with the ligandcomposition. The step of exchanging the surfactants can be achieved byvarious procedures. For example, the surfactants (e.g., carboxylicacids, fatty acids, phosphines and/or phosphine oxides) can be exchangedvia a “mass action” effect, by suspending or dissolving thenanostructures in an organic solvent and combining the suspendednanostructures with the ligand composition, thereby exchanging thesurfactants on the first surface with the ligand composition. Organicsolvents that can be employed for this step include, but are not limitedto, toluene, chloroform, chlorobenzene, and combinations thereof.Alternatively, the surfactants can be removed in situ (e.g., afterdeposition on a substrate) by various techniques, such as performing alow temperature organic stripping procedure followed by oxidation usinga reactive oxygen species (provided, e.g., by UV ozone generation, RFmonoatomic oxygen generation, or oxygen radical generation). The ligandcomposition can then be associated with the stripped nanostructures. Inan alternative class of embodiments, the nanostructures are synthesizedin the presence of the ligand composition, and thus no surfactantexchange step is required.

The methods of the present invention include the step of converting orcuring the ligand composition to generate a second coating (e.g., insome embodiments, a rigid and/or insulating shell) on the first surfaceof the ligand-exchanged nanostructure. In a preferred embodiment, thecuring step is performed by heating the nanostructure having the ligandcomposition associated therewith at temperatures that will not degradeor otherwise compromise the nanostructure. For thenanostructure-containing compositions of the present invention, curingis typically achieved at temperatures less than about 500° C. In someembodiments, the heating process is performed between 200–350° C. Thecuring process results in the formation of the second coating or shell(e.g., a thin, solid matrix on the first surface of the nanostructure).The shell can comprise, for example, an electrically conductivecomposition, an electrically insulating composition, an opticallytransparent composition, an optically opaque composition, or even acombination of these features. In a preferred embodiment, the secondcoating is a rigid insulating shell comprising a glass or glass-likecomposition, such as SiO₂.

The curing step is optionally performed by heating the nanostructure inan oxidizing atmosphere. In embodiments in which the nanostructurecomprises a metal, heating the nanostructure in an oxidizing atmospherecan convert the metal to a metal oxide. The metal oxide is optionallyconverted to the metal by heating the nanostructure in a reducingatmosphere, e.g., after the nanostructure is processed (which caninclude, e.g., exposing the nanostructure to a temperature between about200° C. and about 750° C., or even greater than 750° C.) and/ordisposing a dielectric on the nanostructure.

Optionally, the nanostructures employed in the methods of the presentinvention are coupled to a substrate, e.g., via a second nanostructuresurface. While various substrates can be employed, one exemplarysubstrate is a silicon substrate, e.g., a silicon wafer (e.g., with orwithout a silicon oxide coating). Another exemplary substrate is asilicon nitride surface, either on a silicon wafer, transmissionelectron microscope (TEM) grid, or other suitable substrate. In someembodiments, coated nanostructures are coupled via a secondnanostructure surface (e.g., a portion of the surface not in contactwith the ligand composition).

Optionally, the methods of the present invention further include thestep of applying a planarization composition, e.g., a spin-on glassplanarization composition, to the one or more nanostructures coupled toa substrate. While this optional step can be performed either prior toor after the curing step, the planarization composition is preferablyapplied after curing of the ligand into the rigid shell.

In a further aspect, the present invention provides nanostructureshaving a rigid shell formed post-deposition as prepared by the methodsdescribed herein. In some preferred embodiments, the rigid shellcomprises silicon (for example, SiO2) and/or boron (e.g., B₂O₃). Therigid shell can comprise silicon or silicon oxide. In one class ofembodiments, the rigid shell is a rigid dielectric coating, and theligand composition used to prepare the rigid shell comprises a) a firstcomponent comprising a silicon oxide cage complex, and b) a secondcomponent comprising one or more nanostructure binding moieties, whereineach nanostructure binding moiety is independently coupled to thesilicon oxide cage complex. Each nanostructure binding moiety isoptionally independently coupled to the silicon oxide cage complex viaan oxygen atom. A diameter of the nanostructure is optionally less than6 nm, e.g., less than 3.5 nm.

The present invention also provides methods of reversibly modifyingnanostructures. In the methods, one or more nanostructures comprising ametal are provided. The metal is oxidized to produce a metal oxide, andthe nanostructures are processed. The metal oxide is then reduced toprovide the metal. The metal can be oxidized by heating thenanostructures in an oxidizing atmosphere (e.g., one comprising oxygen).The nanostructures are typically heated to a temperature between about200° C. and about 700° C. (e.g., between about 200° C. and about 500°C.). Similarly, the metal oxide can be reduced by heating thenanostructures in a reducing atmosphere, e.g., an atmosphere comprisinghydrogen, e.g., a forming gas.

These and other objects and features of the invention will become morefully apparent when the following detailed description is read inconjunction with the accompanying figures.

DEFINITIONS

Before describing the present invention in detail, it is to beunderstood that this invention is not limited to particular devices orsystems, which can, of course, vary. It is also to be understood thatthe terminology used herein is for the purpose of describing particularembodiments only, and is not intended to be limiting. As used in thisspecification and the appended claims, the singular forms “a”, “an” and“the” include plural referents unless the content clearly dictatesotherwise. Thus, for example, reference to “a nanostructure” includes acombination of two or more nanostructures; reference to “a ligandcomposition” includes mixtures of ligands, and the like.

Unless defined otherwise, all technical and scientific terms used hereinhave the same meaning as commonly understood by one of ordinary skill inthe art to which the invention pertains. Although any methods andmaterials similar or equivalent to those described herein can be used inthe practice for testing of the present invention, the preferredmaterials and methods are described herein. In describing and claimingthe present invention, the following terminology will be used inaccordance with the definitions set out below.

The term “nanostructure” as used herein refers to a structure having atleast one region or characteristic dimension having a dimension of lessthan about 500 nm, e.g., less than about 100 nm, less than about 50 nm,or even less than about 10 nm or about 5 nm. Typically, the region orcharacteristic dimension will be along the smallest axis of thestructure. Examples of such structures include nanowires, nanorods,nanotubes, branched nanocrystals, nanotetrapods, tripods, bipods,nanocrystals, nanodots, quantum dots, nanoparticles, branched tetrapods(e.g., inorganic dendrimers), and the like. Nanostructures can besubstantially homogeneous in material properties, or in certainembodiments can be heterogeneous (e.g., heterostructures).Nanostructures can be, e.g., substantially crystalline, substantiallymonocrystalline, polycrystalline, metallic, polymeric, amorphous, or acombination thereof. The nanostructures can comprise, e.g., a metal,semiconductor, insulator, or a combination thereof. In one aspect, eachof the three dimensions of the nanostructure has a dimension of lessthan about 500 nm, e.g., less than about 200 nm, less than about 100 nm,less than about 50 nm, less than about 10 nm, or even less than about 5nm.

The terms “crystalline” or “substantially crystalline,” when used withrespect to nanostructures, refer to the fact that the nanostructurestypically exhibit long-range ordering across one or more dimensions ofthe structure. It will be understood by one of skill in the art that theterm “long range ordering” will depend on the absolute size of thespecific nanostructures, as ordering for a single crystal cannot extendbeyond the boundaries of the crystal. In this case, “long-rangeordering” will mean substantial order across at least the majority ofthe dimension of the nanostructure. In some instances, a nanostructurecan bear an oxide or other coating, or can be comprised of a core and atleast one shell. In such instances it will be appreciated that theoxide, shell(s), or other coating need not exhibit such ordering (e.g.it can be amorphous, polycrystalline, or otherwise). In such instances,the phrase “crystalline,” “substantially crystalline,” “substantiallymonocrystalline,” or “monocrystalline” refers to the central core of thenanostructure (excluding the coating layers or shells). The terms“crystalline” or “substantially crystalline” as used herein are intendedto also encompass structures comprising various defects, stackingfaults, atomic substitutions, and the like, as long as the structureexhibits substantial long range ordering (e.g., order over at leastabout 80% of the length of at least one axis of the nanostructure or itscore). In addition, it will be appreciated that the interface between acore and the outside of a nanostructure or between a core and anadjacent shell or between a shell and a second adjacent shell maycontain non-crystalline regions and may even be amorphous. This does notprevent the nanostructure from being crystalline or substantiallycrystalline as defined herein.

The term “monocrystalline” when used with respect to a nanostructureindicates that the nanostructure is substantially crystalline andcomprises substantially a single crystal. When used with respect to ananostructure heterostructure comprising a core and one or more shells,“monocrystalline” indicates that the core is substantially crystallineand comprises substantially a single crystal.

A “nanocrystal” is a nanostructure that is substantiallymonocrystalline. A nanocrystal thus has at least one region orcharacteristic dimension with a dimension of less than about 500 nm,e.g., less than about 200 nm, less than about 100 nm, less than about 50nm, or even less than about 20 nm. The term “nanocrystal” is intended toencompass substantially monocrystalline nanostructures comprisingvarious defects, stacking faults, atomic substitutions, and the like, aswell as substantially monocrystalline nanostructures without suchdefects, faults, or substitutions. In the case of nanocrystalheterostructures comprising a core and one or more shells, the core ofthe nanocrystal is typically substantially monocrystalline, but theshell(s) need not be. In one aspect, each of the three dimensions of thenanocrystal has a dimension of less than about 500 nm, e.g., less thanabout 200 nm, less than about 100 nm, less than about 50 nm, or evenless than about 20 nm. Examples of nanocrystals include, but are notlimited to, substantially spherical nanocrystals, branched nanocrystals,and substantially monocrystalline nanowires, nanorods, nanodots, quantumdots, nanotetrapods, tripods, bipods, and branched tetrapods (e.g.,inorganic dendrimers).

A “substantially spherical nanocrystal” is a nanocrystal with an aspectratio between about 0.8 and about 1.2.

A “nanorod” is a nanostructure that has one principle axis that islonger than the other two principle axes. Consequently, the nanorod hasan aspect ratio greater than one. Nanorods of this invention typicallyhave an aspect ratio between about 1.5 and about 10, but can have anaspect ratio greater than about 10, greater than about 20, greater thanabout 50, or greater than about 100, or even greater than about 10,000.Longer nanorods (e.g., those with an aspect ratio greater than about 10)are sometimes referred to as nanowires. The diameter of a nanorod istypically less than about 500 nm, preferably less than about 200 nm,more preferably less than about 150 nm, and most preferably less thanabout 100 nm, about 50 nm, or about 25 nm, or even less than about 10 nmor about 5 nm. Nanorods can have a variable diameter or can have asubstantially uniform diameter, that is, a diameter that shows avariance less than about 20% (e.g., less than about 10%, less than about5%, or less than about 1%) over the region of greatest variability.Nanorods are typically substantially crystalline and/or substantiallymonocrystalline, but can be, e.g., polycrystalline or amorphous.

A “branched nanostructure” is a nanostructure having three or more arms,where each arm has the characteristics of a nanorod, or a nanostructurehaving two or more arms, each arm having the characteristics of ananorod and emanating from a central region that has a crystal structuredistinct from that of the arms. Examples include, but are not limitedto, nanobipods (bipods), nanotripods (tripods), and nanotetrapods(tetrapods), which have two, three, or four arms, respectively.

A “nanotetrapod” is a generally tetrahedral branched nanostructurehaving four arms emanating from a central region or core, where theangle between any two arms is approximately 109.5 degrees. Typically,the core has one crystal structure and the arms have another crystalstructure.

A “nanoparticle” is any nanostructure having an aspect ratio less thanabout 1.5. Nanoparticles can be of any shape, and include, for example,nanocrystals, substantially spherical particles (having an aspect ratioof about 0.9 to about 1.2), and irregularly shaped particles.Nanoparticles can be amorphous, crystalline, partially crystalline,polycrystalline, or otherwise. Nanoparticles can be substantiallyhomogeneous in material properties, or in certain embodiments can beheterogeneous (e.g. heterostructures). The nanoparticles can befabricated from essentially any convenient material or materials.

An “aspect ratio” is the length of a first axis of a nanostructuredivided by the average of the lengths of the second and third axes ofthe nanostructure, where the second and third axes are the two axeswhose lengths are most nearly equal each other. For example, the aspectratio for a perfect rod would be the length of its long axis divided bythe diameter of a cross-section perpendicular to (normal to) the longaxis.

As used herein, the “diameter” of a nanostructure refers to the diameterof a cross-section normal to a first axis of the nanostructure, wherethe first axis has the greatest difference in length with respect to thesecond and third axes (the second and third axes are the two axes whoselengths most nearly equal each other). The first axis is not necessarilythe longest axis of the nanostructure; e.g., for a disk-shapednanostructure, the cross-section would be a substantially circularcross-section normal to the short longitudinal axis of the disk. Wherethe cross-section is not circular, the diameter is the average of themajor and minor axes of that cross-section. For an elongated or highaspect ratio nanostructure, such as a nanowire or nanorod, a diameter istypically measured across a cross-section perpendicular to the longestaxis of the nanowire or nanorod. For spherical nanostructures such asquantum dots, the diameter is measured from one side to the otherthrough the center of the sphere.

As used herein, the term “coating” refers to a ligand that has beenapplied to a surface, such as the surface of a nanostructure. Thecoating either can fully or partially encapsulate the structure to whichit has been applied. Furthermore, the coating can be porous or solid.

The term “optical property” refers to physical characteristics involvingthe transmission or generation of photons.

Likewise, the term “electrical property” refers to refers to physicalcharacteristics involving the transmission or generation of electrons(or holes).

The phrases “high density packing” or “high density” refer to densitiesof about 10¹² nanostructures per cm² or greater.

An “organic group” is a chemical group that includes at least onecarbon-hydrogen bond.

A “hydrocarbon group” is a chemical group consisting of carbon andhydrogen atoms.

An “alkyl group” refers to a linear, branched, or cyclic saturatedhydrocarbon moiety and includes all positional isomers, e.g., methyl,ethyl, propyl, 1-methylethyl, butyl, 1-methylpropyl, 2-methylpropyl,1,1-dimethylethyl, pentyl, 1-methylbutyl, 2-methylbutyl, 3-methylbutyl,2,2-dimethylpropyl, 1-ethylpropyl, hexyl, 1,1-dimethylpropyl,1,2-dimethylpropyl, 1-methylpentyl, 2-methylpentyl, 3-methylpentyl,4-methylpentyl, 1,1-dimethylbutyl, 1,2-dimethylbutyl, 1,3-dimethylbutyl,2,2-dimethylbutyl, 2,3-dimethylbutyl, 3,3-dimethylbutyl, 1-ethylbutyl,2-ethylbutyl, 1,1,2-trimethylpropyl, 1,2,2-trimethylpropyl,1-ethyl-1-methylpropyl and 1-ethyl-2-methylpropyl, cyclopentyl,cyclohexyl, n-heptyl, n-octyl, 2-ethylhexyl, n-nonyl, n-decyl and thelike. Alkyl groups can be, e.g., substituted or unsubstituted.

An “alkenyl group” refers to a linear, branched, or cyclic unsaturatedhydrocarbon moiety that comprises one or more carbon-carbon doublebonds. Exemplary alkenyl groups include ethenyl, 2-propenyl, 2-butenyl,3-butenyl, 1-methyl-2-propenyl, 2-methyl-2-propenyl, 2-pentenyl,3-pentenyl, 4-pentenyl, 1-methyl-2-butenyl, 2-methyl-2-butenyl,3-methyl-2-butenyl, 1-methyl-3-butenyl, 2-methyl-3-butenyl,3-methyl-3-butenyl, 1,1-dimethyl-2-propenyl, 1,2-dimethyl-2-propenyl,1-ethyl-2-propenyl, 2-hexenyl, 3-hexenyl, 4-hexenyl, 5-hexenyl,1-methyl-2-pentenyl, 2-methyl-2-pentenyl, 3-methyl-2-pentenyl,4-methyl-2-pentenyl, 1-methyl-3-pentenyl, 2-methyl-3-pentenyl,3-methyl-3-pentenyl, 4-methyl-3-pentenyl, 1-methyl-4-pentenyl,2-methyl-4-pentenyl, 3-methyl-4-pentenyl, 4-methyl-4-pentenyl,1,1-dimethyl-2-butenyl, 1,1-dimethyl-3-butenyl, 1,2-dimethyl-2-butenyl,1,2-dimethyl-3-butenyl, 1,3-dimethyl-2-butenyl, 1,3-dimethyl-3-butenyl,2,2-dimethyl-3-butenyl, 2,3-dimethyl-2-butenyl, 2,3-dimethyl-3-butenyl,3,3-dimethyl-2-butenyl, 1-ethyl-2-butenyl, 1-ethyl-3-butenyl,2-ethyl-2-butenyl, 2-ethyl-3-butenyl, 1,1,2-trimethyl-2-propenyl,1-ethyl-1-methyl-2-propenyl, 1-ethyl-2-methyl-2-propenyl, and the like.Alkenyl groups can be substituted or unsubstituted.

An “alkynyl group” refers to a linear, branched, or cyclic unsaturatedhydrocarbon moiety that comprises one or more carbon-carbon triplebonds. Representative alkynyl groups include, e.g., 2-propynyl,2-butynyl, 3-butynyl, 1-methyl-2-propynyl, 2-pentynyl, 3-pentynyl,4-pentynyl, 1-methyl-2-butynyl, 1-methyl-3-butynyl, 2-methyl-3-butynyl,1,1-dimethyl-2-propynyl, 1-ethyl-2-propynyl, 2-hexynyl, 3-hexynyl,4-hexynyl, 5-hexynyl, 1-methyl-2-pentynyl, 1-methyl-3-pentynyl,1-methyl-4-pentynyl, 2-methyl-3-pentynyl, 2-methyl-4-pentynyl,3-methyl-4-pentynyl, 4-methyl-2-pentynyl, 1,1-dimethyl-2-butynyl,1,1-dimethyl-3-butynyl, 1,2-dimethyl-3-butynyl, 2,2-dimethyl-3-butynyl,3,3-dimethyl-1-butynyl, 1-ethyl-2-butynyl, 1-ethyl-3-butynyl,2-ethyl-3-butynyl 1-ethyl-1-methyl-2-propynyl, and the like. Alkynylgroups can be substituted or unsubstituted.

The term “aryl group” refers to a chemical substituent comprising orconsisting of an aromatic group. Exemplary aryl groups include, e.g.,phenyl groups, benzyl groups, tolyl groups, xylyl groups, alkyl-arylgroups, or the like. Aryl groups optionally include multiple aromaticrings (e.g., diphenyl groups, etc.). The aryl group can be, e.g.,substituted or unsubstituted. In a “substituted aryl group”, at leastone hydrogen is replaced with one or more other atoms.

The term “alkyl-aryl group” refers to a group that comprises alkyl andaryl moieties.

A “heteroatom” refers to any atom which is not a carbon or hydrogenatom. Examples include, but are not limited to, oxygen, nitrogen,sulfur, phosphorus, and boron.

A “surfactant” is a molecule capable of interacting (whether weakly orstrongly) with one or more surfaces of a nanostructure.

The term “about” as used herein indicates the value of a given quantityvaries by +/−10% of the value, or optionally +/−5% of the value, or insome embodiments, by +/−1% of the value so described.

A variety of additional terms are defined or otherwise characterizedherein.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts exemplary silsesquioxane frameworks for use asnanostructure ligands in the present invention.

FIG. 2 provides an exemplary discrete silicate ligand having a phosphatemoiety incorporated as a nanostructure binding head group.

FIG. 3 provides a schematic depiction of the preparation of a substrateusing ligand-coated quantum dots. In the top panel, surfactants (crystalsynthesis ligands) coating the surface of a CdSe nanodot are exchangedfor a phosphosilicate ligand. In the middle panel, an SiO₂ surface iscoated with a silane ligand to form a self assembled monolayer ofsurface assembly ligand (SAL). In the bottom panel, the ligand exchangednanodots are applied to the SAL coated substrate, leaving a close-packedmonolayer of CdSe dots on the SiO₂ substrate with SiO₂ between the dotsafter assembly, washing, and curing steps.

FIG. 4 provides a schematic side-view (top) and top-view (bottom)depiction of the conversion of a first coating to a second coating on aplurality of adjacent quantum dots. The views on the left show aclose-packed monolayer of CdSe dots on an SiO₂ substrate with an SiO₂ligand between the dots. Following heat curing, during which the ligandconverts to an SiO₂ dielectric, the views on the right show aclose-packed monolayer of CdSe dots on the SiO₂ substrate with SiO₂between the dots.

FIG. 5 provides exemplary first coating compositions of the presentinvention.

FIG. 6 provides an exemplary synthesis protocol for the production ofthe silsesquioxane ligand heptacyclopentyl POSS disilanoldiethoxyphosphate.

DETAILED DESCRIPTION

Many electronics applications would benefit from processes andcompositions that provided nanostructures having improved energy barrierheights and/or quantum confinement. Nanostructures having these enhancedproperties could be used, e.g., for quantized charge storage and/ortransfer in the field of microelectronics, or for photon generation andtransfer in photonics. For example, solid state storage devices such asflash memory devices use storage media having discrete read and writeproperties. Enhanced storage capacities could be implemented by storingcharge on densely-packed discrete nanostructures, such as quantum dots.In particular, nanostructures that pack well at high density (e.g.,those having spherical, nearly spherical, and/or isotropic structures,such as nanodots or quantum dots) as well as improved quantumconfinement properties are particularly promising for use in discreteand/or quantized charge storage, as well as for photon generation andtransfer.

Cross-talk between dots (i.e., signal interference due to electronicinteractions between the nanostructures) leads to poor deviceperformance. The present invention, however, provides compositions,methods and devices in which nanostructured charge storage elements areable to be closely packed (e.g. at densities of 1×10¹⁰/cm² or greater,even at a high density, e.g., at 1×10¹²/cm² or greater), whilepreserving or improving quantum confinement, either by controlling thedistance between the nanostructures and/or by introducing an insulatingor dielectric coating material such as silicon dioxide around discretenanostructures.

For example, two significant issues considered with respect to the useof nanostructures as charge storage elements are the inclusion ofappropriate surface properties, and the packing of the selectednanostructures into ordered or disordered monolayers. For high-densitydata storage applications, the nanostructures are preferably provided asone or more close-packed ordered monolayers. In the case ofsemiconducting nanocrystals, hexagonally packed monolayers of CdSe havebeen prepared in the art by making use of phase segregation betweenaliphatic surfactants on the nanocrystals and aromatic conjugatedorganic materials, and deposition via spin-coating. However, theembedding of nanocrystals into (or on top of) an organic matrix is notdesirable in memory device fabrication processes. To this end, thepresent invention provides, in one embodiment, monolayers of quantumdots with silsesquioxane or silicate ligand surface ligands prepared byvarious self-assembly methods and compatible with charge storageapplications.

Maintaining a selected distance between nanostructures can be achievedusing a ligand or coating associated with the nanostructure surface. Thesize of the ligand-nanostructure complex, and thus the distance betweenadjacent nanostructures, can be varied for different applications byaltering the composition of the associated ligand. Thus, the size of theligand can be used to control dot-to-dot spacing during the preparationof a nanostructure-containing substrate or matrix.

In addition, the physical properties of the nanostructure compositioncan also be adjusted by introducing a ligand coating that can beconverted to a second coating having a second, desired property (forexample, being dielectric). For example, in some embodiments providedherein, the coated nanocrystals in their “post-processing” or curedstate are insulated with silicon dioxide-containing second coatings orshells, e.g., to reduce cross-talk between nanocrystals. Other desirableproperties include, but are not limited to, malleability, rigidity,thermal tolerance, conductivity, transparency, and opaqueness (opacity),depending upon the application involved. Furthermore, ligandcompositions that, upon conversion to a second coating, affect the HOMOor valence bond levels of the nanostructure composition are alsoincluded in the compositions of the present invention.

However, while primarily described in terms of charge insulation and/ornanostructure spacing for, e.g., charge storage applications such asnon-volatile memory, it will be appreciated by those of skill in the artupon reading the present disclosure that the present invention, and orvarious individual or combined component aspects thereof, possess farbroader applicability than that which is embodied by these specificapplications. In particular, the ability to provide or include aconvertible coating that can be converted in situ, or otherwise whendesired (e.g., after association with the nanostructure, so as to alterthe property of the nanostructure), has broadly applicable value. Forexample, optical coatings may be deposited using a coating material thatoffers a first optical property, but which may be converted to a secondoptical property, post-deposition. Additionally, the ability toindividually associate a coating with a nanostructure, which coating maybe more easily manipulated in one form, but may be later converted whilealready uniformly or otherwise desirably coated onto the nanostructure,provides significant advantages to previously described nanostructurecoating processes.

Discrete Coated Nanostructures

The present invention provides methods and compositions involvingdiscrete coated nanostructures. These nanostructures differ fromnanostructures embedded in a matrix, in that each coated nanostructurehas, upon synthesis or after subsequent application, a defined boundaryprovided by the coating that is not contiguous with the surroundingmatrix. For ease of discussion, the coating material is generallyreferred to herein as a “ligand” in that such coating typicallycomprises molecules that have individual interactions with the surfaceof the nanostructure, e.g., covalent, ionic, van der Waals, or otherspecific molecular interactions. The present invention also provides aplurality of discrete coated nanostructures, in which the first coatingshave been converted to the second coatings such that the individualnanostructures are not in direct contact or otherwise in undesirablecommunication, e.g., electrical communication. Furthermore, the secondcoating (shell) component of the coated nanostructure is oftennon-crystalline, unlike the typical core:shell type nanostructures knownin the art. Optionally, the diameters of the coated nanostructures(e.g., the nanostructure:coating construct) are less than about 10 nm,and optionally less than about 5 nm, less than about 4 nm, or even lessthan about 3.5 nm.

A discrete coated nanostructure of the present invention includes anindividual nanostructure having a first surface and a first coatingassociated with the first surface of the individual nanostructure andhaving a first optical, electrical, physical or structural property,wherein the first coating is capable of being converted to a secondcoating having a different electrical, optical, structural and/or otherphysical property than the first coating. In some embodiments, the firstcoating encapsulates the nanostructure (i.e., it completely surroundsthe nanostructure being coated). In other embodiments, the nanostructureis partially encapsulated. For example, the first coating can cover theportion of the nanostructure not associated with another composition,such as the surface of a substrate.

Pluralities of Coated Nanostructures

The present invention also provides a coated nanostructure-containingcomposition having a plurality of nanostructures having a first coatingseparating each member nanostructure. Typically the coating has aplurality of nanostructure binding moieties that are employed to attachthe coating to the surface of the member nanostructures. The firstcoating then can be converted to a second coating or shell thatpossesses at least one different property from the original coating,e.g., a coating that is electrically, optically, chemically, and/orstructurally different, e.g., insulative as opposed to conductive (or atleast non-insulative), or rigid instead of malleable. An insulatingcoating (or insulating shell) as described herein comprises a materialthat is nonconductive (e.g., dielectric). An insulating shell isgenerally capable of preventing substantial charge transfer for at leasta brief length of time; for example, the insulating shell can reduce therate of charge diffusion between member nanostructures, such that theaverage time for an electron to hop from one member nanostructure toanother is at least a millisecond, or optionally at least 10milliseconds, at least 100 milliseconds, at least 1 second, at least 1minute, at least 1 hour, at least 1 day, at least 1 month, or at least 1year or longer. Optionally, the charge transfer is substantiallyprevented (e.g., a device comprising the insulated nanostructures canmaintain an applied charge) for a predetermined length of time rangingfrom 1 millisecond to at least 1 second, 1 minute, 1 hour, 1 day, 1year, or longer. By providing a convertible coating mechanism inaccordance with the present invention, e.g., as opposed to a synthesizednanocrystal that includes a shell component, one can garner a number ofadvantages, including, e.g., providing smaller core-shell structures,and potentially more coherent shell layers, that allow higher packingdensities when such nanocrystals are arranged in a layer, e.g., amonolayer. For some embodiments, providing the plurality ofnanostructures at a density of about 1×10¹⁰/cm² is sufficient. However,in preferred embodiments, the plurality of nanostructures in thenanostructure-containing composition layer are present at a density ofabout 1×10¹¹/cm² or greater, or about 1×10¹²/cm² or greater, and morepreferably, at about 1×10¹³/cm² or greater.

Optionally, the plurality of discrete coated nanostructures (e.g., at aselected density) are provided as a monolayer. However, in someembodiments, the plurality of nanostructures includes multiplemonolayers, each independently having a selected or desired density ofmember nanostructures.

In a preferred embodiment, the plurality of coated nanostructuresfunction as charge storage elements in various high-density data storageapplications. Two key requirements for the use of the plurality ofcoated nanostructures in these applications are the selection ofappropriate surface properties, and close packing of the nanostructuresin monolayer arrays, optionally well-ordered monolayer arrays. As shownby Bulovic and coworkers (Coe et al. 2002 “Electroluminescence fromsingle monolayers of nanocrystals in molecular organic devices” Nature420:800–803), hexagonally-packed monolayers of CdSe-type semiconductingnanocrystals can be prepared by taking advantage of phase segregationbetween aliphatic surfactants on the nanocrystals and aromaticconjugated organic materials deposited on the nanocrystal viaspin-coating. However, a composition of nanocrystals embedded into (oron top of) a 40 nm thick organic matrix is not desirable in memorydevice fabrication processes. Among other issues, the thickness of the(fairly-conductive) organic matrix will not provide enough quantumconfinement, and will reduce the read/write efficacy and predictabilityof the device. Furthermore, the organic layer(s) are not compatible withtypical memory fabrication techniques. To this end, coatednanostructures which are more compatible with charge storageapplications are provided by the present invention. In a specificpreferred embodiment, the plurality of coated nanostructures of thepresent invention comprise one or more monolayers of nanodots havingsilsesquioxane or silicate ligand surface ligands. These can beprepared, for example, by various self-assembly methods as describedherein; after curing, the resulting nanostructures are insulated by thesecond coating of silicon dioxide-containing ligands. Among otheradvantages, the oxide second coating reduces cross-talk betweennanostructures.

Coatings and Related Properties

The ligands employed as first coatings in the compositions, devices andmethods of the present invention are prepared as a means by which togenerate a second coating having a selected or desired property (orproperties). The second coating provides an altered electrical, optical,physical or structural state as compared to the first coating, such aschanges in rigidity, solubility, and/or in optical properties(refractive index, emission and/or absorption properties). A variety ofcoating compositions are considered for use in the present invention.For example, the coating can be an organic composition, such as variouspolymeric precursors that may be chemically or radiatively converted toaltered (second) coating compositions, e.g., through cross-linking,further polymerization, etc. Exemplary organic compositions include, butare not limited to, dendrimer PAMAM (amine dendrimer), amine-(or othernanocrystal binding head group) terminated methyl methacrylate(polymethylmethacrylate precursor), phosphonate head group-containingpolymers, carboxylic acid-terminated diene or diacetylene compositions,any heteroatom containing monomer(s) that can be converted to polymersupon chemical, heat or light activation, as well as the ligandsdescribed in by Whiteford et al. in U.S. Ser. No. 10/656,910 filed Sep.4, 2003, and titled “Organic Species that Facilitate Charge Transferto/from Nanostructures.”

Alternatively, the coating is an inorganic composition. Optionally, thecoating includes a silicon or silicon oxide moiety. It will beunderstood by one of skill in the art that the term “silicon oxide” asused herein can be understood to refer to silicon at any level ofoxidation. Thus, the term silicon oxide can refer to the chemicalstructure SiO_(x), wherein x is between 1 and 2 inclusive. Inorganiccoatings for use in the present invention include, but are not limitedto, tin oxide, vanadium oxide, manganese oxide, titanium oxide,zirconium oxide, tungsten oxide, and niobium oxide, silicon carbide,silicon nitride, as well as other silicon-containing coatings and/orboron-containing coatings. In some preferred embodiments, the coatingcomprises a hybrid organic/inorganic composition, such as someembodiments of the silicon oxide cage complexes provided herein. Seealso the compositions provided in Schubert (2001) “Polymers Reinforcedby Covalently Bonded Inorganic Clusters” Chem. Mater. 13:3487–3494;Feher and Walzer (1991) “Synthesis and characterization ofvanadium-containing silsesquioxanes” Inorg. Chem. 30:1689–1694; Coronadoand Gomez-García (1998) “Polyoxometalate-Based Molecular Materials”Chem. Rev. 98:273–296; Katsoulis (1998) “A Survey of Applications ofPolyoxometalates” Chem. Rev. 98:359–387; Muller and Peters (1998)“Polyoxometalates” Very Large Clusters—Nanoscale Magnets” Chem. Rev.98:239–271; Rhule et al (1998) “Polyoxometalates in Medicine” Chem. Rev.98:327–357; Weinstock (1998) “Homogeneous-Phase Electron-TransferReactions of Polyoxometalates” Chem. Rev. 98:113–170; and Suzuki (1999)“Recent Advanced in the Cross-Coupling Reactions of OrganoboronDerivatives with Organic Electrophiles 1995–1998” J. Organomet. Chem.576:147–168; Sellier et al. (2003) “Crystal structure and charge orderbelow the metal-insulator transition in the vanadium bronze β-SrV₆O₁₅ ”Solid State Sciences 5:591–599; Bulgakov et al. (2000) “Laser ablationsynthesis of zinc oxide clusters: a new family of fullerenes?” Chem.Phys. Lett. 320:19–25; Citeau et al. (2001) “A novel cageorganotellurate (IV) macrocyclic host encapsulating a bromide anionguest” Chem. Commun. Pp. 2006–2007; Gigant et al. (2001) “Synthesis andMolecular Structures of Some New Titanium (IV) Aryloxides” J. Am. Chem.Soc. 123:11623–11637; Liu et al. (2001) “A novel bimetallic cage complexconstructed from six V₄Co pentatomic rings: hydrothermal synthesis andcrystal structure of [(2,2′-Py₂NH)₂Co]₃V₈O₂₃ ” Chem. Commun. Pp.1636–1637; and “On the formation and reactivit of multinuclearsilsesquioxane metal complexes” 2003 dissertation thesis of Rob W. J. M.Hanssen, Eindhoven University of Technology.

In a preferred embodiment, the coating is a silicon-containing coating(e.g., either an inorganic or hybrid inorganic/organic composition) thatcan be converted to a rigid SiO₂ insulating shell after deposition ofthe coating and association of the nanostructure binding moieties withthe surface of the member nanostructure. The present invention providescoated nanostructures in which the second coating comprises a rigid SiO₂shell, and wherein a diameter of the discrete coated nanostructure isoptionally less than or equal to 50 nm, less than or equal to 20 nm,less than or equal to 10 nm, less than or equal to 6 nm, or less than orequal to 3.5 nm.

In some embodiments, the coating can be used to provide spacing betweenadjacent member nanostructures, e.g., during preparation ofsubstrate-bound nanostructure compositions (see, for example, theembodiment depicted in FIGS. 3 and 4). Optionally, the coating ligandsof the present invention are sized such that the coated nanostructurescan be packed to provide less than about 10 nm between nanostructures(center to center), or optionally less than about 8 nm, less than about5 nm, or less than about 4 nm between nanostructure centers. In manyembodiments, the coating provides a spacing of between about 8–10 nm,about 4–8 nm, or preferably about 2–4 nm between nanostructure surfaces(e.g., the ligands are 1–2 nm in length).

In a preferred embodiment, the coating composition or the rigid shellreduces or prevents charge diffusion between member nanostructures.Coating compositions that can be converted into second coatings ofoxides of silicon and/or boron are particularly preferred in thisembodiment.

Optionally, after conversion of the ligand coating to a second coating(one that typically has differing properties than the first coating),the coated nanostructures are associated with a substrate and/oroverlaid with a topcoat material. Optionally, the top coating materialis a similar composition to that of either the first coating or secondcoating. For example, after formation of rigid SiO₂ shells arounddiscrete nanostructures, a plurality of the nanostructures can beoverlaid with a composition that can also be converted to SiO₂, thusembedding the nanostructures in a matrix of silicon.

The ligands employed as first coatings in the compositions, devices andmethods of the present invention are prepared as a means by which togenerate a second coating having a selected or desired property (orproperties). For example, quantum dots used in flash memory devices needto maintain discrete boundaries between adjacent nanostructures. Thiscan be achieved by providing a ligand that can be converted to a rigidshell (second coating) having a defined diameter, thus controlling thedistance between dots. In addition, device performance can be improvedif the second coating also functions to improve quantum confinement andreduce cross-talk between quantum dots; a ligand that produces a secondcoating that has dielectric characteristics is also desirable. Thepresent invention provides ligand compositions for use as firstcoatings, for use in the generation of discrete coated nanostructureshaving e.g., improved barrier heights and/or quantum confinement.

The first coating and second coating typically have differing physicalproperties. For example, the first coating can be electrically neutral(the first electrical property) while the second coating comprises adipole moment (the second electrical property); similarly, the firstcoating can comprise a dipole moment while the second coating iselectrically neutral. In another embodiment, the first coating isnon-insulating or conductive (e.g., a conjugated conductingorganic-metal hybrid species), while the second coating is insulating ornonconductive (e.g., a metal oxide). In a further embodiment, the firstcoating is insulating or nonconductive, and the second coating isnon-insulating or conductive. Of particular interest are malleable firstcoatings that are converted to rigid second coatings (particularly thosehaving semiconductive or insulating properties). One preferredcomposition embodiment for use as a rigid insulating shell encapsulatingthe selected nanostructure is silicon oxide (SiO₂); such rigid SiO₂second coatings are optionally produced from malleable first coatingscomprising silicon oxide caged complexes (e.g., silsesquioxanes).

Alternatively, the first and second coatings may differ in opticalproperties. For example, the first optical property comprises lightabsorption or emission at a first wavelength, and the second opticalproperty comprises light absorption or emission at a second wavelength(e.g., by a lanthanide-containing coating or the like). Alternatively,the first optical property could be reduced or non-transmission of light(opaqueness) while the second optical property is transparency (or viceversa). Another embodiment of interest includes first and secondcoatings that have different bandgap energies, e.g., to alter theelectron and/or conductivity properties of the coated nanostructure.

As another example, the first and second coatings can differ in aphysical property such as solubility, e.g., in a selected solvent. Forexample, the first coating can render the coated nanostructures solublein a selected solvent, to facilitate dispersal, deposition, or the likeof the nanostructures, while nanostructures including the second coatingare less soluble in the selected solvent. It will be evident that thefirst and second coatings can have combinations of the above properties;for example, the first coating may increase solubility in a selectedsolvent, while the second coating is nonconductive.

Silicon Oxide Cage Complexes

In a preferred embodiment, the ligand coating used to coat thenanostructures is a silicon oxide cage complex. The polycyclicsilicon-containing compounds known as silsesquioxanes (orsilasesquioxanes), e.g., polyhedral oligomeric silsesquioxanes (POSS),are one type of soluble discrete silicon oxide cage complex (see, forexample, Hanssen supra). Exemplary silsesquioxanes include hydrogensilsesquioxane (HSQ) and methyl silsesquioxane (MSQ); additionalsilsesquioxane structures are provided in FIG. 1 (in which the R groupsinclude a variety of chemical moieties, including, but not limited to,short chain alkyl groups such as methyl, ethyl, isopropyl, isobutyl,longer chain alkyl groups such as isooctyl and norbornyl, as well asaromatic and non-aromatic cyclic structures such as phenyl, cyclopentyl,cyclohexyl and cycloheptyl groups. The silsesquioxane can be either aclosed cage structure or a partially open cage structure (e.g., in whichsome of the ring oxygens are not coupled to both adjacent silicon atoms;see for example, FIG. 5B). The non-silicate organic group, which islocated along an edge or at a corner of the cage complex, can befunctionalized to accommodate binding of the ligand to an exposedsurface of the nanostructure. Optionally, the non-silicate group canfunction as an electron withdrawing (or electron donating) group.Functional groups which can be incorporated into the silsesquioxanemoiety include, but are not limited to, alkyl, alcohol, phosphine,phosphonate, thiol, ether, carboxylate, amine, epoxide, alkene and arylgroups, as well as other nanostructure binding moieties, solubilizingmoieties, or electron withdrawing/donating groups of interest.

One preferred derivatization is the incorporation of boron into thesilicon oxide cage monomer, which, will produce a second coating ofboron oxide and silicon oxide upon heat treatment.

Exemplary silsesquioxane frameworks are provided in FIG. 1.Silsesquioxanes can be either purchased or synthesized, for example, byhydrolytic condensation of RSiCl₃ or RSi(OR)₃ monomers (see, forexample, Feher et al. (1989) J. Am. Chem. Soc. 111:1741; Brown et al.(1964) J. Am. Chem. Soc. 86:1120; Brown et al. (1965) J. Am. Chem. Soc.87:4313–4323). The nature of the caged structures formed duringsynthesis (e.g., type of polyhedral, closed versus open) can be directedby manipulation of the reaction conditions including solvent choice, pH,temperature, and by the choice of R-group substituent (Feher et al.(1995) Polyhedron 14:3239–3253). Additional silsesquioxane frameworks(e.g., for derivatization with nanostructure binding moieties) areavailable from Hybrid Plastics (Fountain Valley, Calif.; on the worldwide web at hybridplastics.com).

Typically, the silsesquioxane frameworks are coupled to one or morenanostructure binding moieties prior to use as compositions or in themethods of the present invention. Any of a number of standard couplingreactions known in the art can be used to derivatize the silsesquioxaneframework, e.g. with one or more nanostructure binding head groups. See,for example, the reactions described in Feher et al. (1995) Polyhedron14:3239–3253. Additional information regarding general synthesistechniques (as known to one of skill in the art) can be found in, forexample, Fessendon and Fessendon, (1982) Organic Chemistry, 2nd Edition,Willard Grant Press, Boston Mass.; Carey & Sundberg, (1990) AdvancedOrganic Chemistry, 3rd Edition, Parts A and B, Plenum Press, New York;and March (1985) Advanced Organic Chemistry, 3rd Edition, John Wiley andSons, New York. Optionally, the standard chemical reactions describedtherein are modified to enhance reaction efficiency, yield, and/orconvenience.

Silsesquioxane compositions for use as first coatings in the presentinvention include (but are not limited to) the compositions provided inFIG. 5 and Table 1.

Additional discrete silicates can also be derivatized with nanostructurebinding moieties to form compositions of the present invention. Forexample, cyclopentyltrimethoxysilane (CAS 143487-47-2) will condensewith water and assemble into cage structures. The nanostructure bindinghead group can then be coupled to one or more of the free hydroxylpositions, either before or after cage formation.

Phosphosilicate ligands are another preferred embodiment for use in thecompositions and methods described herein. As depicted in FIG. 2, thephosphate group on the phosphosilicate ligand can be utilized to couplethe ligand to a nanostructure. Preferably, phosphosilicate ligands thatcould be thermally decomposed into SiO₂ are utilized in the methods andcompositions of the present invention; shells incorporating SiO₂ wouldlead to higher barrier height than ZnS, and potentially highertemperature tolerances during subsequent processing or manufacturingsteps. Exemplary phosphosilicate ligands are provided in FIG. 5, panelsA and B.

Additional ligands having thiol moieties as the nanostructure bindinghead groups are depicted in FIG. 5, panels D–I. It will be evident thatcertain nanostructure binding groups are preferred for certainnanostructure compositions; for example, ligands having thiol (e.g.,aryl thiol) moieties are preferred ligands for certain metalnanostructures (e.g., Pd nanostructures).

Exemplary nanostructure binding moieties, one or more of which istypically independently coupled to the silicon oxide cage complex via anoxygen or silicon atom, include, but are not limited to: the protonatedor deprotonated forms of phosphonate, phosphinate, carboxylate,sulfonate, sulfinate, amine, alcohol, amide, and/or thiol moieties,ester moieties of phosphonate, phosphinate, carboxylate, sulfonate, andsulfinate, phosphines, phosphine oxides, and epoxides.

Polyoxometalates

In other embodiments of the present invention, the ligand coating usedto coat the nanostructures is a polyoxometalate. Polyoxometalates aremetal-oxygen cluster anions, typically formed from early transitionmetals (V, N, Ta, Mo and W) in their highest oxidation state. Numerousderivatives can be prepared from polyoxometalate compositions, includinghalide, alkoxyl, thiol, phospho, and organosilyl derivatives; for a goodreview, see Gouzerh and Proust (1990) Chem. Rev. 98:77–111. For example,polyoxovandanate derivatives can be used as first coatings in thecompositions and methods of the present invention. The first ligandswould then be converted to a second coating comprising vanadium oxide,having properties comparable to those of silicon oxides.

The polyoxometalates can be used as a first coating on thenanostructure, and subsequently converted to a second coating havingdiffering properties. Certain polyoxometalates (for example, acid formsof molybdenum and tungsten-based polyoxometalates) have photochromic orelectrochromic properties, which can be reduced or altered uponconversion to a second coating (e.g., by treatment with an organicreducing agent, or by exposure to an externally applied electric field(see, for example, Yamase (1998) Chem. Rev. 98:307–325)).

Other Ligand Compositions

Optionally, the second ligand includes a catechol functional group,which can be used to tune the electrochemical properties of the secondcoating. Catechol functional groups for use in the present inventioninclude, but are not limited to, pyrocatechol, salicylic acid, and2,2-biphenol (see, for example, Gigant et al. (2001) J. Am. Chem. Soc.123:11632–11637).

In many embodiments of the present invention, the second coating is aninsulating composition (e.g., used to form an insulating shell aroundthe nanostructure). In a preferred embodiment, the second coating is ametal oxide, or a glass or glass-like composition capable of formingoxide polyhedra. Silicon dioxide (SiO₂), boron oxide (B₂O₃), andtitanium oxide (TiO₂) are preferred second coatings components that canbe generated from the first coatings of the present invention by, e.g.,thermal degradation (although other oxidation states can also beemployed). Other second coatings of interest include, but are notlimited to, compositions including GeO₂, P₂O₅, AsO₅, P₂O₃, As₂O₃, Sb₂O₃,V₂O₅, Nb₂O₅, Ta₂O₅, SnO₂ and WO₃, as well as other oxidation states ofthe provided metal oxides.

Exemplary Compositions

Exemplary compositions for use as the first coating in the presentinvention are provided in Table 1 below, as well as in FIGS. 5 and 6.

TABLE 1 Compound 1

where R is a cyclopentyl group Compound 2

where R is a cyclopentyl group Compound 3

where R is a hydrogen or alkyl group Compound 4

where R is an alkyl group, a heteroatom, or an electron withdrawinggroup Compound 5

where R is an alkyl group or a nanostructure binding group Compound 6

where R is a hydrogen, an alkyl group, or a nanostructure binding groupCompound 7

where R is a halide, a leaving group, or a nanostructure binding groupCompound 8

wherein R is an isobutyl group Compound 9

where R is an isobutyl group Compound 10

where R is an alkyl group or a hydrogen atom Compound 11

where R is an alkyl group Compound 12

where R is an isobutyl group Compound 13

where R is a cyclohexyl groupOther exemplary compositions for use as the first coating include, butare not limited to, compounds like Compounds 1–3,5–6, and 8–13, butwhere R is an organic group or a hydrogen atom. For example, R can be ahydrocarbon group. In certain embodiments, R is an alkyl group (e.g., acyclic alkyl group or a short alkyl group having fewer than 20 or evenfewer than 10 carbon atoms), an aryl group, an alkylaryl group, analkenyl group, or an alkynyl group. For example, in some embodiments, Ris an isobutyl group, a methyl group, a hexyl group, a cyclopentylgroup, or a cyclohexyl group.

In one aspect, the present invention also provides compositions forindividually coating discrete nanostructures with a dielectric coating.The composition includes a first component comprising a silicon oxidecage complex and a second component comprising one or more nanostructurebinding moieties, wherein each nanostructure binding moiety isindependently coupled to the silicon oxide cage complex, e.g., via anoxygen or silicon atom. The compositions of the present invention areconverted to the dielectric coating after deposition of the compositionon a surface of the nanostructure.

Nanostructures

Nanostructures prepared by any of a number of synthetic techniques knownin the art can be used to prepare a discrete coated nanostructure of thepresent invention, including both semiconductor and metallicnanostructures, for example. Typically, the first coating is convertedto the second coating after completion of synthesis of thenanostructure, e.g., after the nanostructures have been removed from anysolvents or building materials used during the synthesis process.Preferably, the first coating is not difficult to displace from thenanostructure surface.

Optionally, the nanostructures are associated with the surface of asubstrate, such as a silicon wafer or a TEM grid. In some embodiments,the substrate has been treated with a composition for association withthe nanostructures, such as a functionalized self-assembly monolayer(SAM) ligand. Exemplary compositions for functionalizing the substratesurface include a silicon nitride coating, a silane ligand having ananostructure binding moiety, or other chemical moiety that can provideor accept a proton for hydrogen-bonding to the coated nanostructure(e.g. amine, alcohol, phosphonate, fluorine or other non-carbonheteroatom). For example, the silane ligand can include structureshaving the formula [X₃Si-spacer-binding group(s)] where X is a Cl, OR,alkyl, aryl, other hydrocarbon, heteroatom, or a combination of thesegroups, and where the spacer is an alkyl, aryl and/or heteroatomcombination. Optionally, the structure of the ligand can be responsiveto light activation, leading to crosslinking of ligands (e.g., to eachother, or the surface of the SAL coated substrate) via inclusion of aphoto-crosslinkable group. Exemplary surface ligands for use in thepresent invention (referred to generically as “SAL” in FIG. 4) arecommercially available from Gelest Inc. (Tullytown, Pa.; on the worldwide web at gelest.com).

The individual nanostructures employed in the compositions include, butare not limited to, a nanocrystal, a nanodot, a nanowire, a nanorod, ananotube, a quantum dot, a nanoparticle, a nanotetrapod, a tripod, abipod, a branched nanocrystal, or a branched tetrapod. The presentinvention is not limited to either semiconductor nanostructures ormetallic nanostructures; the type of nanostructure employed isdetermined in part by the purpose for which it is intended. While any ofthese nanostructure embodiments can be used in the present invention,spherical, nearly spherical, and/or isotropic nanocrystals such asnanodots and/or quantum dots are used as the prototypical nanostructurefor illustration purposes. For many embodiments, the diameter (e.g., afirst dimension) of the coated nanodot or quantum dot is less than about10 nm, and optionally less than about 8 nm, 6 nm, 5 nm, or 4 nm. In someembodiments, the nanostructure (e.g., dot) diameters ranges from about 2nm to about 4 nm. In a preferred embodiment for use with densely-packednanostructure arrays, the diameter of the coated quantum dot or nanodotis less than or equal to about 6 nm, or optionally less than or equal toabout 3.5 nm.

Nanostructures, such as nanocrystals, quantum dots, nanoparticles andthe like, can be fabricated by a number of mechanisms known to one ofskill in the art. Furthermore, their size can be controlled by any of anumber of convenient methods that can be adapted to different materials,and they are optionally washed to remove excess surfactants remainingfrom their synthesis and/or excess ligands. See, for example, U.S.patent applications U.S. Ser. No. 10/796,832 to Scher et al. titled“Process for producing nanocrystals and nanocrystals produced thereby,”filed Mar. 10, 2004; U.S. Ser. No. 60/544,285 to Scher et al. titled“Methods of processing nanocrystals, compositions, devices and systemsusing same,” filed Feb. 11, 2004; U.S. Ser. No. 60/628,455 to Scher etal. titled “Process for group III–V semiconductor nanostructuresynthesis and compositions made using same,” filed Nov. 15, 2004; andU.S. Ser. No. 60/637,409 to Whiteford, et al. titled “Process for group10 metal nanostructure synthesis and compositions made using same,”filed Dec. 16, 2004; and references therein.

The nanostructures employed in the nanostructure-containing compositionsof the present invention can be fabricated from essentially anyconvenient materials. For example, the nanocrystals can compriseinorganic materials, e.g., a semiconducting material selected from avariety of Group II–VI, Group III–V, or Group IV semiconductors, andincluding, e.g., a material comprising a first element selected fromGroup II of the periodic table and a second element selected from GroupVI (e.g., ZnS, ZnO, ZnSe, ZnTe, CdS, CdSe, CdTe, HgS, HgSe, HgTe, MgS,MgSe, MgTe, CaS, CaSe, CaTe, SrS, SrSe, SrTe, BaS, BaSe, BaTe, and likematerials); a material comprising a first element selected from GroupIII and a second element selected from Group V (e.g., GaN, GaP, GaAs,GaSb, InN, InP, InAs, InSb, and like materials); a material comprising aGroup IV element (Ge, Si, and like materials); a material such as PbS,PbSe, PbTe, AlS, AIP, and AlSb; or an alloy or a mixture thereof. Metalssuch as Pd, Pt, Au, Ag, Ni, Fe, Sn, Zn, Ti, Ir, and Co can also be usedin the synthesis of nanostructures for use in the present invention, ascan metal oxides. Further details regarding nanocrystalline structuresfor use in the present invention can be found, for example, U.S. patentapplication Ser. No. 10/656,802, filed Sep. 4, 2003, titled“Nanocomposite Based Photovoltaic Devices” and incorporated herein byreference in its entirety for all purposes.

In a preferred embodiment, the devices of the present invention employnanostructures comprising small, roughly spherical CdSe or Pdnanocrystals, or other metal or semiconductor-based nanostructures thatcan be synthesized as spherical, nearly spherical, and/or isotropicnanoparticles (such as nanodots and/or quantum dots).

Methods for Post-Deposition Shell Formation on a Nanostructure

Methods for making and using core/shell CdSe/ZnS semiconductors preparedvia deposition on or in a layer of conducting organic material are knownin the art, but these methods present several problems. For example, thethin ZnS shell of the nanostructure:shell construct does not have a highenough energy barrier to prevent leakage of the charge from of thenanostructure. While this problem can be addressed by growing a verythick ZnS shell, this approach is synthetically impractical, as afterseveral monolayers, the strain causes defect formation, the nanocrystalsbecome insoluble, and the spacing between the nanocrystals would be toolarge to meet the packing density desired for memory applications. Theproblem could theoretically be addressed by growing a core structure(CdSe) having a first shell (ZnS) and an additional shell (SiO₂),however, this approach would also have the same disadvantages withrespect to defect formation, solubility and spacing. The presentinvention circumvents these problems, either by performing a ligandexchange directly onto the selected nanostructure using a ligand thatcan be turned into a second coating (for example, an oxide) upon curingbut will maintain the nanostructure solubility in organic solvents(e.g., for deposition purposes), or by growing the nanostructures in thepresence of such a ligand.

The present invention provides methods for post-deposition shellformation on a nanostructure. These methods include the steps of a)providing one or more nanostructures having a ligand compositionassociated with a first surface, which a ligand composition is capableof being converted to a second coating having differing electrical,optical, physical or structural properties (e.g., to a rigid shell), andb) curing the ligand composition and generating the second coating(e.g., the rigid shell) on the first surface of the nanostructure,thereby forming a shell on the nanostructure post-deposition of theligand composition on the nanostructure. The methods of the presentinvention are preferably performed at temperatures that do notcompromise or degrade the structural and/or physical properties of thenanostructure.

In one class of embodiments, the nanostructures having the ligandcomposition associated therewith are provided by exchanging surfaceligands. In this class of embodiments, providing one or morenanostructures having a ligand composition associated with a firstsurface comprises providing one or more nanostructures having one ormore surfactants associated with the first surface and exchanging thesurfactants on the first surface with the ligand composition. In anotherclass of embodiments, the nanostructures are synthesized in the presenceof the ligand composition, and no ligand exchange is necessary.

Providing the Nanostructures

The methods of the present invention can be used to generate a shell orsecond coating on any of a number of nanostructures, including, but notlimited to, a nanocrystal, a nanodot, a nanowire, a nanorod, a nanotube,a quantum dot, a nanoparticle, a nanotetrapod, a nanotripod, ananobipod, a branched nanostructure, and the like. Furthermore, themethods of the present invention are not limited to nanostructuresprepared by a specific synthetic approach. For example, organometallicsolution-based syntheses of Pd, CdSe, CdTe and CdS nanocrystalstypically employ various surfactants and/or fatty acids as solubilizingagents (see, for example, U.S. patent publication 2002/0066401 to Penget al. titled “Synthesis of colloidal nanocrystals,” U.S. patentpublication 2003/173541 to Peng et al. titled “Colloidal nanocrystalswith high photoluminescence quantum yields and methods of preparing thesame,” Kim et al. (2003) NanoLetters 3:1289–1291, and Qu et al. (2001)NanoLetters 1:333–337, and references cited therein). Nanostructuresprepared using these or other weakly-binding organic compositions can beemployed in the methods of the present invention.

Exchanging Surface Ligands

In some embodiments of the methods, the nanostructures are provided bypreparing or growing the initial structures (e.g., the corenanostructure components) in the presence of a weakly binding organiccomposition (the “growth ligand”). The growth ligand has a weakerassociation with the nanostructure than the ligand used to generate thefirst coating (a “replacement ligand”), and thus can be readilyexchanged, e.g., by mass action.

The nanostructures employed in the methods of the present inventioncommonly have one or more organic compositions, or growth ligands,associated with the nanostructure surface (e.g., for solubilizing thenanostructure during the synthesis procedure). Typical growth ligandsinclude surfactants, for example, phosphines or phosphine oxides such astrioctyl phosphine (TOP), tri-n-butyl phosphine (TBP), or trioctylphosphine oxide (TOPO) or acids such as hexadecyl phosphonic acid (HDPA)or octadecyl phosphonic acid (ODPA). Alternatively or in addition,various long chain carboxylic acids (e.g., fatty acids, such as stearic,palmitic, myristic, lauric, capric, caprylic, caproic and butyric acids,as well as other saturated or nonsaturated lipid-like structures) mayhave been employed during synthesis and remain associated with thenanostructure surface. In the methods of the present invention, thegrowth ligands are exchanged for a ligand composition capable of beingconverted to a second ligand or second coating having a differentelectrical, optical, physical or structural property, thereby forming aligand-exchanged nanostructure composition. In a preferred embodiment,the growth ligands are exchanged for a ligand composition capable ofbeing converted to a rigid insulating shell, such as an oxide.

Exchanging the surfactants associated with the nanostructure surfacewith a ligand or first coating of the present invention can be achievedby any of a number of mechanisms known in the art. In one embodiment,exchanging the surfactants involves suspending or dissolving thenanostructures in an organic solvent, and combining the suspendednanostructures with the ligand composition. Solvents that can be usedfor the exchange process include any that are typically employed inconjunction with nanostructure synthesis and processing, such astoluene, chloroform, chlorobenzene, and the like. The temperature atwhich the exchanging step is performed will depend upon the ligandsinvolved and may range from room temperature to elevated temperaturesequal or greater than 100° C., 200° C., 300° C. and the like. Forexample, surface ligands comprising sulfonic acid moieties can beexchanged without substantial heating, and optionally can be performedat room temperature.

In another embodiment, the nanostructures are coupled to or associatedwith a substrate surface (e.g., a solid phase embodiment rather than insolution). The organic surfactants on the nanostructure surface can beremoved in situ, for example, via a low temperature organic strippingprocess (at temperatures <500° C., and optionally between 200–350° C.).The stripping process is optionally followed by oxidation using, e.g., areactive oxygen species. The replacement ligand (e.g., the ligand of thefirst coating) is subsequently applied to the nanostructure by any of anumber of techniques known in the art (vapor deposition, spraying,dipping, etc.).

Self Assembly of Monolayers

Optionally, the ligand coated nanostructures are induced to formmonolayers due to intermolecular self-assembly forces. For example, in apreferred embodiment, the present invention provides nanocrystals withsilsesquioxane or silicate ligands tailored for charge storageapplications. Preferably, the nanostructures are arranged into closepacked arrays, or more preferably high density and/or orderedclose-packed arrays. Controlled self-assembly of the close-packed arrayscan be achieved by various wet-process methods, such as deposition ofthe nanostructure-first ligand composition onto self-assembledmonolayers (SAMs) or otherwise functionalized substrates or oxides, orby evaporation-driven assembly.

The member components of the self-assembled monolayer associate withboth the surface of the substrate as well as the nanostructure, thusforming a bridge or linker between the two. Various SAM compositions foruse in the present invention include, but are not limited to,organosilanes, phosphonic acids, phosphines, thiols, amines,heteroatoms, and the like. In one preferred embodiment, the SAM consistsof a silane ligand with a binding head for the silsesquioxane orsilicate ligand. In an alternate preferred embodiment, the substratesare directly functionalized with binding groups suitable for binding tothe nanocrystals. The nanostructures are applied in a solution anddeposited on the SAM or functionalized substrate by, e.g., spin-coating,dip-coating, spray-coating, or conventional printing technologies. Theexcess (unbound) nanostructures are subsequently washed off thesubstrate using an organic solvent such as toluene or chloroform,resulting in a monolayer of nanocrystals coated with silicon-containingligands.

Alternatively, the monolayers can be prepared by evaporation-drivenassembly, without the need of specially treated substrates. Thenanocrystals are deposited on the substrate from solution byspin-coating, dip-coating, spray-coating, or conventional printingtechnologies. By controlling the de-wetting process of the solvent,well-ordered arrays of nanocrystals can be obtained.

Further details regarding monolayer formation can be found, for example,in U.S. patent application Ser. No. 60/671,134 to Heald et al. titled“Methods and devices for forming nanostructure monolayers and devicesincluding such monolayers,” filed Apr. 3, 2005, incorporated herein byreference in its entirety for all purposes.

Curing the Ligand Composition and Generating the Second Coating

After deposition and monolayer formation, the substrate can be thermallyannealed to cure the layer of first coating (and thereby form the secondlayer, which in some embodiments is a rigid insulating shell, on thefirst surface of the nanostructure). The technique used for the curingstep will depend upon the type of ligand composition employed in themethod. The curing can be done under inert atmosphere, such as argon ornitrogen, or under oxygen, for example. The temperature of the curingprocess can be adjusted for the surface ligands. For example, curing thecomposition can involve heating the nanostructure having the ligandcomposition associated therewith to form the rigid shell on thenanostructure surface. Heating can be performed in one or more stages,and using various equipment such as a hot plate or quartz furnace (seeYang et al (2001) Proc. Natl. Acad. Sci. 25:339–343). In someembodiments, the ligand:nanostructure complex is heated to less thanabout 500° C., and optionally, to between 200–350° C. Thermal curing ofsilsesquioxane ligands typically involves heating thesilsesquioxane-containing composition to temperatures of less than about500° C., and preferably less than about 350° C., thereby transformingthe cage structures into a network structure. In other embodimentsinvolving silicon-containing ligands, the thermal curing processdecomposes the first coating into a second coating of SiO₂. Conversionof the first coating to the second coating (or shell) can be monitored,for example, via thermogravimetric analysis using an FTIR spectrometer(see Yang (2001) supra, and references cited therein).

In alternate embodiments, conversion of the ligand composition from thefirst coating to a second coating or shell having altered electronic oroptical properties can include irradiating the composition. For example,for embodiments employing PMMA precursors or carboxylate diene ordiacetylene moieties, the polymerization process is light activated,leading to crosslinkage of the first coating to form the organic shell(second coating).

In some embodiments, the one or more nanostructures provided in themethods of the present invention are coupled to a substrate via a secondnanostructure surface. Optionally, this substrate is a silicon wafer. Insome embodiments, the member nanostructures are encapsulated prior toassociation with the substrate surface, while in other embodiments, afirst portion of a member nanostructure is associated with thesubstrate, and a second portion of the member nanostructure isassociated with the first coating or the second coating. Optionally, thesurface of the silicon wafer includes a silane ligand coupled to asecond nanostructure binding moiety, e.g., to facilitate association ofthe substrate with a portion of the nanostructure surface.

The curing process is optionally followed by spin coating of anotherlayer of e.g., first coating, silicate, or the like, onto thesubstrate-bound coated nanostructures, and thermal curing, therebyproviding a top coating or overlay. In some embodiments, the top layeris an insulating oxide layer. The methods of the present inventionoptionally further include the step of applying a planarizationcomposition as the overlay or top coating composition applied to thesubstrate-coupled nanostructures. The optional planarization compositioncan be applied either before or after the step of curing the ligandcomposition. The planarization composition fills any remaining narrowspaces and produces a (relatively) flat surface on the treated portionof the wafer and/or nanostructure composition. Preferably, the topcoating or planarization material is compatible with the rigid shell ofthe coated nanostructure. Optionally, the planarization composition is adielectric material (either similar or different in composition from thesecond coating composition).

Exemplary planarization materials include, but are not limited to,various silicates, phosphosilicates, and siloxanes referred to as SpinOn Glass (SOG). Optionally, the ligand compositions of the presentinvention can be used as the planarization composition.

The present invention also provides nanostructures having a rigid shellformed post-deposition as prepared by the methods described herein. In apreferred embodiment, the rigid shell comprises silicon or siliconoxide, and the diameter of the nanostructure:shell composition is lessthan or equal to about 6 nm.

Method for Reducing Charge Diffusion Among a Plurality of Quantum Dots

In a further aspect, the present invention provides methods for reducingcharge diffusion among a plurality of nanostructures, e.g., nanodots,and particularly quantum dots. The methods include the steps of couplinga ligand composition comprising an electron withdrawing group to asurface of a member nanodot (or quantum dot or other nanostructure), andforming a dipole on the surface of the member nanodot and increasing theelectron affinity of the nanodot, thereby reducing any charge diffusion(such as lateral charge diffusion) among the nanodots. Optionally, thenanostructures thus formed are used in the compositions and methods forpost-deposition shell formation as described herein.

Many of the ligand compositions of the present invention have electronwithdrawing characteristics and can be utilized as electron-withdrawingcompositions in the present methods (e.g., silicon oxide cage complexessuch as silsesquioxanes). In some embodiments, the electron withdrawingcomposition includes a fluorine atom (for example, F, SiF, an SiFderivative, or a fluorine polymer such as polytetrafluoroethylene). Inother embodiments, the ligand composition is a boron-containingcomposition (e.g., an aryl-boron oligomer or a boronic acidcomposition). Optionally, the electron withdrawing composition includesa nanostructure binding group, such as a phosphonic acid moiety,phosphonate ester, or other nanostructure binding moiety such as thosedescribed herein, for coupling to the nanostructure surface.

Optionally, the first and second properties of the ligand compositionsof the present invention are photochromism-related properties (e.g.,involving color changes induced in the coating by an incoming stimulus,such as light or other incident electromagnetic radiation). In someembodiments, the electron withdrawing composition comprises alight-activated intramolecular salt, e.g., a spiropyran. Exemplaryintramolecular salts for use in the methods and compositions of thepresent invention include, but are not limited to,HOOCCH₂CH(NH(CH₃)₂)CH₂CH₂PO₃H₂. See also Léaustic et al. (2001)“Photochromism of cationic spiropyran-doped silica gel” New. J. Chem.25:1297–1301 and references cited therein.

In one class of embodiments, the plurality of nanodots (or quantum dotsor other nanostructures) comprises discrete quantized photon generationand transfer media or discrete quantized charge storage or chargetransfer media.

The present invention also provides one or more (e.g., a plurality of)nanodots (for example, quantum dots) or other nanostructures havingreduced charge diffusion, as prepared by the methods described herein.The nanostructures optionally have a rigid shell formed post-depositionof the ligand composition, e.g., a rigid shell comprising silicon orsilicon oxide. The nanostructures can be of essentially any material,size, and/or shape. In one preferred class of embodiments, a diameter ofthe nanostructures is less than 6 nm, e.g., less than 3.5 nm.

Additional details regarding suitable ligand compositions for modifyingnanostructure properties can be found, e.g., in U.S. patent application60/635,799 by Whiteford et al. entitled “Compositions and methods formodulation of nanostructure energy levels,” filed Dec. 13, 2004.

Methods for Fabricating a Memory Device

The present invention also provides methods for fabricating ananostructure-based memory device that uses the nanocrystals to storecharge. As described in Coe et al. 2002, supra, core/shell CdSe/ZnSsemiconductors can be deposited on/in a layer of conducting organicmaterial. However, there are several problems with this previouslydescribed method. First, the thin ZnS shell generated by this methoddoes not have a high enough energy barrier to prevent leakage of thecharge out of the nanocrystal. While this problem could theoretically beaddressed by growing a very thick ZnS shell, this approach issynthetically impractical. After deposition of several monolayers ofshell, the strain causes defect formation, and/or the nanocrystalsbecome insoluble, thereby providing a practical limitation to feasibleshell thickness. Furthermore, the spacing between the thickly coatednanocrystals would be too large to meet the packing density desired formemory applications. The problem might also be addressed by growing acore (CdSe) shell (ZnS) and a third shell (SiO2), an approach that issynthetically feasible but has similar issues as to those listed above.The present invention takes the novel approach of performing a ligandexchange directly onto the nanostructure (for example, small, roughlyspherical CdSe or Pd nanocrystals) using a ligand composition asprovided herein (e.g., a modified silsesquioxane ligand). (Alternately,as noted, the nanostructure can be grown in the presence of the ligandcomposition.) Preferably, the first coating of ligand can be convertedor cured into an oxide, and will maintain the nanostructure solubilityin organic solvents for deposition purposes.

The methods for fabricating a nanostructure-based memory device thatuses the nanocrystals to store charge include the steps of a) providinga plurality of nanostructures the members of which have associatedtherewith a weakly binding growth ligand; b) exchanging the growthligand with a replacement ligand and forming a first coating on themember nanostructures; c) associating the coated member nanostructureswith a surface of a substrate; and d) converting the first coating to asecond coating that differs in one or more electrical, optical, physicalor structural properties, thereby fabricating a nanostructure-basedmemory device. In a related class of embodiments, steps a and b arereplaced by a single step, in which the nanostructures are synthesizedin the presence of the ligand, whereby the ligand forms a first coatingon the member nanostructures. Preferably, nanoparticles havingspherical, nearly spherical, and/or isotropic geometries (such asnanodots and/or quantum dots) are most effective for close packing ofthe nanostructures. Exchanging the growth ligand or surfactant for areplacement ligand of the first coating can be done, for example, bymass action exchange. To facilitate this process, the binding constantsfor the weakly bound growth ligand are preferably less than those of theligand for use in the first coating.

One advantage to this approach to nanostructure synthesis is that thenanostructure product contains fewer organic contaminants than thoseprepared by methods currently available. Another advantage is that thelength of the replacement ligand can be tuned to control the diameter ofthe coated nanostructure and thus properly space the nanocrystals apartto reduce and/or prevent charge leakage, while still allowing highdensity packing.

Devices

Many electronic and optical applications can be manufactured using thenanostructure-containing compositions of the present invention.Particularly, any device that employs (or can be devised to employ)nanodot nanostructures would benefit from the compositions and methodsof the present invention. For example, various electronic applicationssuch as transistors and memory devices could be prepared using thenanostructure-containing compositions of the present invention. Lightemitting applications, such as LEDs, back plane lighting for LCDs,phosphors, PVs, photodetectors, and photodiodes could also employ thenanostructure-containing compositions of the present invention, as couldother optoelectronic devices such as photovoltaic devices. Furthermore,the coated nanostructures could be employed in signal dampeningcompositions and/or as detectable labels (e.g., based upon a secondoptical property having a specified emission wavelength.)

The nanostructure-containing compositions of the present invention areparticularly useful for the construction of flash memory constructs.Flash memory is a type of electrically-erasable programmable read-onlymemory (EEPROM) that can be rapidly erased and reprogrammed. Devicesutilizing this type of constantly-powered, nonvolatile memory canoperate at higher effective speeds than standard EEPROM devices, sincethe memory is altered in blocks, instead of one byte at a time.

Flash memory typically encodes a single bit per cell, which comprisestwo transistors (a control gate and a floating gate) separated by a thinoxide layer. The cell is characterized by the specific threshold voltagebetween the two gates. Electrical charge is programmed/stored on thefloating gate, which also controls the two possible voltage levelsbetween the transistors (the on/off status of the cell). Multi-bittechnology is also being developed, in which the cells have two or morevoltage thresholds (i.e., the voltage across each cell has been dividedinto greater than two levels). Additional details of nanostructure-basedmemory devices, transistors, and the like can be found, e.g., in U.S.patent application Ser. No. 11/018,572 by Xiangfeng Duan et al. entitled“Nano-enabled memory devices and anisotropic charge carrying arrays”,filed Dec. 21, 2004.

As noted herein, unregulated signal transmission between proximal signalcarriers (cross-talk) reduces the performance/efficiency of a givendevice. One mechanism by which cross-talk among nanostructures in ananostructure-containing device can be reduced is by increasing thedistance between the nanostructures. This approach is particularlyuseful when dealing with nanoscale structures such as quantum dots.Increasing the distance between adjacent quantum dots can beaccomplished by forming a rigid shell encompassing each member dot,thereby controlling the distance between them. The rigid shell is formedafter deposition of a first coating onto the discrete nanostructures,thereby maintaining the discrete (physically separate) character of theindividual nanostructures. If made out of an appropriate (e.g.,dielectric or nonconductive) material, the rigid shell can also provideanother mechanism for reducing cross-talk between nanostructures.

The nanostructure-containing compositions of the present invention canbe prepared at densities of 10¹⁰/cm², 10¹¹/cm², 10¹²/cm², or greaterwithout loss of quantum confinement or increased cross-talk betweenmember quantum dots.

The present invention provides novel processes for producingheterostructural nanocrystals, e.g., nanocrystals that are comprised oftwo or more different compositional elements where the differentelements together impart useful properties to the nanocrystals. As notedherein, such heterostructures are typically embodied in a core-shellorientation, where a core of a first material is surrounded by a shellof a second material. It is worth noting that the first material cancomprise a conductor, a semiconductor, or an insulator (e.g., adielectric), and the second material can likewise comprise a conductor,a semiconductor, or an insulator (e.g., a dielectric), in any possiblecombinations (e.g., two conductive materials, a conductive material andan insulator, etc.). The methods of the present invention provideflexibility of processing to allow more facile fabrication of thesenanocrystals, as well as manipulation of certain parameters, e.g. sizesin the sub-10 nm range, that were previously not attainable. As aresult, it is expected that any application to which typical core-shellnanocrystals were to be put would be a potential application for thecompositions of the present invention, e.g., those nanocrystalcompositions made in accordance with the processes described herein. Inaddition, a variety of additional applications will be enabled by theabilities that are gained from these novel processes.

Methods for Reversible Modification of Nanostructures

For some applications, e.g., fabrication of certain nanostructure-baseddevices, nanostructures must withstand high temperature processing,e.g., without melting and fusing with adjacent nanostructures. Althoughnanostructures comprising a material with a high melting point can beselected for use in such applications, all materials have their meltingpoint lowered as the physical size of a structure is reduced to thenanometer range; high temperature processing steps can thus beproblematic even for high melting point materials.

The present invention provides novel processes for reversibly modifyingnanostructures, e.g., nanostructure components of semiconductor devices,to protect the nanostructures from subsequent process steps. As onespecific example, the methods of the invention can be used to oxidizepalladium quantum dots (e.g., by a high temperature anneal in anoxidizing atmosphere), increasing their resistance to fusion during theprocess of encapsulating the dots in an overlying dielectric whilefabricating a flash memory device. The oxidation can be reversed (e.g.,by a high temperature anneal in a reducing atmosphere) to convert thepalladium oxide back to pure (or substantially pure) palladium, tocapitalize on the properties of palladium metal for device performance.It is worth noting that the methods of the invention can protectnanostructures of any of variety of materials, shapes, and sizes duringa variety of subsequent manipulations, including but not limited toexposure to high temperatures.

One general class of embodiments thus provides methods of reversiblymodifying nanostructures. In the methods, one or more nanostructurescomprising a metal are provided. The metal is oxidized to produce ametal oxide, and the nanostructures are processed. The metal oxide isthen reduced to provide the metal.

The metal can be oxidized by heating the nanostructures in an oxidizingatmosphere (e.g., one comprising oxygen). The nanostructures aretypically heated to a temperature between about 200° C. and about 700°C. (e.g., between about 200° C. and about 500° C.). Similarly, the metaloxide can be reduced by heating the nanostructures in a reducingatmosphere, e.g., an atmosphere comprising hydrogen, e.g., a forming gas(i.e., 5% H₂ in N₂). It will be evident that the reactive gas(es) arepreferably able to access the nanostructures through any material(s)surrounding the nanostructures. Alternatively, the nanostructures can beat least partially reduced by heating in a nitrogen atmosphere. Thenanostructures are typically heated to a temperature between about 200°C. and about 700° C. (e.g., between about 200° C. and about 500° C.).

The nanostructures to be modified can be of essentially any size and/orshape. Thus, for example, the nanostructures can include one or morenanowires, nanorods, nanotubes, branched nanocrystals, nanotetrapods,tripods, bipods, nanocrystals, nanodots, quantum dots, nanoparticles,branched tetrapods, or a combination thereof. In one class ofembodiments, the nanostructures are substantially sphericalnanostructures.

The methods can be used for nanostructures comprising any metal that canundergo reversible oxidation. For example, the metal can be a noblemetal (e.g., Au, Ag, or Pt) or a transition metal (e.g., Ni, Fe, Sn, orZn). In one preferred class of embodiments, the metal is Pd; in thisclass of embodiments, the metal oxide is typically PdO. The entirenanostructure or a portion thereof (e.g., a surface layer) can beoxidized. For example, greater than 10% of the metal comprising apopulation of nanostructures can be oxidized, e.g., greater than 20%,greater than 50%, greater than 75%, or even greater than 90%. Oxidation(and, conversely, reduction) can be monitored, e.g., via a techniquesuch as energy dispersive spectrometry (EDS).

As noted, such reversible oxidation can protect nanostructures duringprocessing, e.g., certain device fabrication steps that are performed athigh temperature. Thus, for example, in one class of embodiments,processing the nanostructures comprises exposing the nanostructures to atemperature between about 200° C. and about 750° C. (e.g., a temperaturegreater than about 250° C., greater than about 500° C., or greater thanabout 600° C.), or even to a temperature greater than about 750° C. Suchelevated temperatures can be encountered, for example, when disposing adielectric on the nanostructures.

The nanostructures can be protected, e.g., from fusion at hightemperature, by reversible oxidation. Additionally (or alternatively),the nanostructures can be protected by a coating such as those describedherein. Thus, in one class of embodiments, the one or morenanostructures provided have a first coating associated with a firstsurface of each nanostructure. The first coating has a first optical,electrical, physical or structural property, and is capable of beingconverted to a second coating having a different optical, electrical,physical or structural property. The first and/or second coatings canbe, e.g., any of those described herein. Thus, for example, the secondcoating can comprise an oxide, e.g., SiO₂, optionally formed from asilsesquioxane composition such as those described herein. The firstcoating can be converted to the second coating by heating thenanostructures in an oxidizing atmosphere; it will be evident that theconversion can be simultaneous with oxidation of the metal. The coating(e.g., SiO₂) can help maintain physical separation between thenanostructures and thus reduce the tendency for adjacent nanostructuresto fuse when exposed to high temperatures. Silsesquioxane ligandscontain substoichiometric oxygen for formation of SiO₂; curing a firstcoating comprising a silsesquioxane in an oxidizing atmosphere can thusform a better quality SiO₂ second coating, which can also (oralternatively) assist in blocking nanostructure fusion.

EXAMPLES

The following examples are offered to illustrate, but not to limit theclaimed invention. It is understood that the examples and embodimentsdescribed herein are for illustrative purposes only and that variousmodifications or changes in light thereof will be suggested to personsskilled in the art and are to be included within the spirit and purviewof this application and scope of the appended claims.

Example 1 Preparation of Closely Packed Nanostructure Monolayers

A method for preparing a substrate having closely packed nanostructuresis depicted schematically in FIGS. 3 and 4. A nanodot (depicted as asphere) is synthesized with surfactants that coat the surface. Thesurfactants are ligand-exchanged for the silsesquioxane or othersilicate ligand (L).

A selected substrate (e.g., a silicon dioxide wafer) is coated with asilane ligand bearing a nanostructure binding head group (B). The silaneligands interact and associate into a self assembled monolayer ofsurface assembly ligand (SAL) on the substrate surface, providing ananostructure-binding interface (as indicated by the perpendiculararrows). An exemplary surface assembly ligand includes a cyclic dimethylamino moiety and a SiMe₂ group coupled together via a linker (cyclicdimethyl amino-organic spacer-SiMe₂).

The ligand exchanged nanodots are then put on the SAL substrate by spincoating or dip coating with the solvent containing the dots. The excessdots are washed off the substrate, resulting in a monolayer of nanodotsinsulated with silicon dioxide containing ligands. Due to the monolayernature of the surface assembly ligand, the nanodots are closely packed(shown in side view in FIG. 4). The nanostructure-bound substrate isthen thermally annealed to cure the layer, thus converting the firstcoating (for example, a phosphosilicate ligand) into a second coating (ashell of SiO₂). The resulting annealed surface is optionally treated tospin coating of another layer (a topcoat or overlay) of silicate andthermal curing, to produce a nanodot memory device.

Example 2 Synthesis of Heptacyclopentyl POSS Disilanol Diethoxyphosphate

Synthesis of the exemplary polyhedral oligomeric silsesquioxane (POSS)ligand heptacyclopentyl POSS disilanol diethoxyphosphate 2 was performedas provided herein (FIG. 6). All procedures were carried out under aninert atmosphere using Schlenk technique. The solvents were dried over 4Å molecular sieves and degassed with three freeze-vacuum-thaw cycles.The heptacyclopentyl POSS trisilanol 1 was dried by static vacuum in adessicator with phosphorous pentoxide for 12 hours, and diethylchlorophosphonate (Cl—P(O)(OEt)₂ was vacuum transferred before use. Massspectrometry was performed at Scripps Research Institute in La Jolla,and ³¹P {¹H} NMR spectroscopy was performed with a Bruker FT NMR using³¹P at 162 MHz.

The reaction was set up in a 50 mL Schlenk flask. Heptacyclopentyl POSStrisilanol 1 (1.00 g, 1.14 mmol) was dissolved in a combination oftoluene (10 mL) and triethylamine (15 mL) and produced a clear solution.Then ClP(O)(OEt)₂ (0.650 g, 0.545 mL, 3.77 mmoles) was added by syringewhile stirring over 1 minute. After about 5 minutes, the clear solutionturned cloudy. It was stirred overnight under argon.

Approximately 20 hours after the addition of ClP(O)(OEt)₂, the volatilecomponents were removed by vacuum transfer. The residue was extractedwith hexane (3×8 mL) and the volatiles removed again by vacuum transfer.The residue was dissolved in 1.25 mL of toluene and precipitated out ofsolution as an oil with 6 mL of acetonitrile. The upper phase wasdiscarded and the precipitation process repeated twice. Then the oil wasdissolved in 6 mL of THF, 2 mL of toluene and eventually about 6 mL ofacetonitrile. The last solvent was added slowly with mixing until thesolution turned cloudy. Then the mixture was cooled to −35° C.overnight, which produced some white micro-crystals. The supernatant wasremoved and the volatile solvents removed by vacuum transfer until atabout one third of the original starting volume remained, thus providinga substantial quantity of white micro-crystals. The remainingsupernatant was removed leaving the product in the flask. Then the whitecrystalline product 2 was dried under vacuum until a pressure of <0.010torr was attained for 1 hour. The product was isolated as whitemicro-crystals 0.320 g, 0.313 mmol or 27.5% yield. Mass Spec: ESI-TOF(−)m/z 1034 [M−H+Na], ESI-TOF(+) m/z 1011 [M−H]. NMR ³¹P{¹H} NMR (162 MHz,Tol-d₈, 25 C) δ-11.3 (s, 1P).

This reaction also works with 2.0 equivalents of Cl—P(O)(OEt)₂ and 2.0eq Et₃N or pyridine in toluene. The reaction procedure was performed asdescribed above, including the hexane washes, and the product wasisolated by crystallization at −35° C. from a mixed solvent systemconsisting of THF, toluene and acetonitrile.

Other silsesquioxane derivatives of the present invention include:

-   1) Closed Silicate Cage POSS molecule mono-silanol, having an    organic spacer bonded to the alcohol to give an ether (aryl or alkyl    derivatives), and a carbon bond on other end of the spacer leading    to the nanostructure binding head group.-   2) Open Silicate Cage POSS molecule tri-silanol, having three    organic spacers bonded to alcohols to give a tri-ether, and the    carbon bond on the other end of the spacer linking to the    nanostructure binding moiety.-   3) Silicate dimer (or larger oligomer) compound prepared by    condensation. Difunctional Silane and mono-heteroatom functionalized    POSS, having a binding group centered at middle of the difunctional    Silane spacer unit.-   4) Conversion of silicate closed cage from endo to exo by selective    (Si—O—Si) opening of the cage (e.g., on one side) and modification    of exposed di-ol with the binding head group, for side access    binding or cross-linking cage molecules.

Example 3 Generation of a Monolayer of Coated Nanostructures On a SAM

The controlled self-assembly of monolayers of nanocrystals withsilsesquioxane or silicate ligands tailored for charge storageapplications can be achieved by various wet-process methods, such as thedeposition onto self-assembled monolayers (SAMs). This approach can beused to prepare monolayers having close packed nanostructure arrays, andpreferably ordered close-packed nanostructure arrays.

A self assembled monolayer consisting of a silane ligand with a bindinghead for the silsesquioxane or silicate ligand is applied to a substratesurface. The nanocrystals are deposited on the SAM from solution byspin-, dip-, or spray-coating, or conventional printing technologies.The excess dots are washed off the substrate resulting in a monolayer ofnanocrystals insulated with silicon dioxide containing ligands.

Example 4 Generation of an Ordered Monolayer of Coated Nanostructures byEvaporation-Driven Assembly

The nanostructure-containing monolayers of the present invention canalternatively be prepared by evaporation-driven assembly. In thisembodiment, specially-treated substrates functionalized or layered withchemical moieties for associating with the nanostructure are notrequired. CdSe nanocrystals are drop-cast onto a silicon nitridesubstrate. The dewetting process is controlled by the composition of thesurface ligand and by wicking the surface with a solvent-absorbingcleanroom cloth. By controlling the de-wetting process of the solvent,well-ordered arrays of nanocrystals can be obtained.

Example 5 Preparation of Arrayed Nanostructures for Use in MemoryDevices

The present invention describes a general approach to making a memorydevice based on using nanocrystals for charge storage. The method wasreduced to practice using CdSe nanocrystals without a shell, which werethen ligand-exchanged with a silsesquioxane ligand modified with aphosphonate ester head group to bind to the nanocrystal. Thesenanocrystals were then deposited on an oxide-coated substrate inmonolayers.

The same general approach used, however, could be readily applied tometal nanocrystals by modifying the nanocrystal synthesis to makeroughly spherical metal nanocrystals with weak binding ligands, forexample Pd nanocrystals. These would then be cleaned and characterized,e.g., via NMR. The ligand would be modified by attaching a differenthead group to the silsesquioxane, for example a thiol or sulfonate groupto better bind to the nanocrystal. The ligand would be purified, andthen characterized by NMR and mass spectrometry. The ligand would beexchanged onto the nanocrystal using VT-NMR to monitor the exchange. Theexchanged nanocrystals would then be cleaned to remove excess ligand.The nanocrystals will then be deposited via spin-coating or evaporationonto the prepared substrate (SAM coated, functionalized, orunfunctionalized oxide substrate).

Various aspects of the present invention can be readily varied oraltered while still accomplishing the synthesis of discrete coatednanostructures. The type of nanostructures employed can be varied: CdSe,any II–VI, III–V, or group IV semiconductor, any metal (including, butnot limited to, Pd, Pt, Au, Ag, Ni, Fe, Sn, Zn, and Co). A narrow sizedistribution can be provided either during the initial synthesis, or bysubsequent size selection. Furthermore, the ligand binding group foreither the weakly-bound growth ligand or the first coating (e.g.,oxide-related) ligand can be varied: thiol, sulfonate, sulfinate,phosphinate, carboxylate, phosphonate, phosphonate ester, amine,phosphine, etc. Various oxide ligands can be generated (upon curing)depending upon the selection of first coating and intended use, such asSiO_(x), TiO_(x), VnO_(x) or other oxides. The method of deposition canalso be varied beyond those described here.

Another method for forming an oxide would be to controllably oxidize thenanocrystal surface (for example, by bubbling oxygen through a dilutesolution of nanocrystals) to produce an oxide that provides an energybarrier (for example, a Co core with a cobalt oxide shell). The firstcoating ligands of the present invention could still be applied insolution and cured after deposition of the monolayer. The approachapplied in this memory application could also be used for nanocrystalsthat need to be embedded in a matrix, such as tagants or phosphors.

Example 6 Preparation of a Nanostructure-Based Charge Storage Device

Nanocrystal-based capacitors can be prepared, e.g., as a demonstrationof the feasibility of nanocrystal-based charge storage devices such asflash memory devices. To fabricate such an example device, a siliconwafer with a 3–6 nm thick tunnel oxide layer on it is prepared.Palladium quantum dots having a ligand composition of the invention(e.g., the POSS ligand illustrated in FIG. 5 Panel F) associatedtherewith are prepared by surfactant exchange or by synthesis in thepresence of the ligand and suspended in an organic solvent such astoluene. The nanocrystals are then spun or dropped onto the surface ofthe oxide-coated wafer, wet, and dried down. Excess nanocrystals arerinsed off, leaving basically a monolayer of nanocrystals on the wafer.The wafer is baked in an atmosphere comprising oxygen at 250° C. for10–30 minutes to cure the ligand composition and form the second coating(e.g., an SiO₂ shell). Another oxide layer (e.g., an SiO₂ layer) isdeposited on the nanocrystals by chemical vapor deposition, and chromeand gold are evaporated onto the oxide layer to form an electrode. Thedevice can then be characterized by measuring CV curves before and afterapplying program and erase voltages.

While the foregoing invention has been described in some detail forpurposes of clarity and understanding, it will be clear to one skilledin the art from a reading of this disclosure that various changes inform and detail can be made without departing from the true scope of theinvention. For example, all the techniques and apparatus described abovecan be used in various combinations. All publications, patents, patentapplications, and/or other documents cited in this application areincorporated by reference in their entirety for all purposes to the sameextent as if each individual publication, patent, patent application,and/or other document were individually indicated to be incorporated byreference for all purposes.

1. A discrete coated nanostructure comprising: an individualnanostructure coated with a first coating, wherein the first coating hasa first optical, electrical, physical or structural property, andwherein the first coating is convertible to a second coating having adifferent optical, electrical, physical or structural property than thefirst coating.
 2. The coated nanostructure of claim 1, wherein the firstcoating or the second coating encapsulates the nanostructure.
 3. Thecoated nanostructure of claim 1, wherein a diameter of the discretecoated nanostructure is between about 2 nm and about 6 nm.
 4. The coatednanostructure of claim 1, wherein the individual nanostructure comprisesa nanocrystal, a nanodot, a nanowire, a nanorod, a nanotube, a quantumdot, a nanoparticle, a nanotetrapod, a nanotripod, a nanobipod, or abranched nanostructure.
 5. The coated nanostructure of claim 1, whereinthe second coating comprises an oxide.
 6. The coated nanostructure ofclaim 5, wherein the second coating comprises SiO₂.
 7. The coatednanostructure of claim 6, wherein the first coating comprises: a firstcomponent comprising a silicon oxide cage complex; and a secondcomponent comprising one or more moieties that bind to thenanostructure, wherein each moiety is independently coupled to thesilicon oxide cage complex.
 8. The coated nanostructure of claim 7,wherein each moiety is independently coupled to the silicon oxide cagecomplex via an oxygen atom.
 9. The coated nanostructure of claim 7,wherein the silicon oxide cage complex comprises a silsesquioxanecomposition.
 10. The coated nanostructure of claim 9, wherein thesilsesquioxane comprises a closed cage structure.
 11. The coatednanostructure of claim 9, wherein the silsesquioxane comprises apartially open cage structure.
 12. The coated nanostructure of claim 9,wherein the silsesquioxane is derivatized with one or more boron,methyl, ethyl, isopropyl, isobutyl, branched or straight chain alkanehaving between 3 and 22 carbons, branched or straight chain alkenehaving between 3 and 22 carbons, phenyl, cyclopentyl, cyclohexyl,cycloheptyl, isooctyl, norbornyl, or trimethylsilyl groups, or acombination thereof.
 13. The coated nanostructure of claim 7, whereinthe silicon oxide cage complex comprises one or more discrete silicates.14. The coated nanostructure of claim 13, wherein the discrete silicatecomprises a phosphosilicate.
 15. The coated nanostructure of claim 7,wherein the moiety comprises one or more of the protonated ordeprotonated forms of phosphonate, carboxylate, amine, phosphinate,phosphonate ester, sulfonate, sulfinate, alcohol, amide, or thiolmoieties.
 16. The coated nanostructure of claim 6, wherein the firstcoating is selected from the group consisting of:

where R is an organic group or a hydrogen atom.
 17. The coatednanostructure of claim 16, wherein R is a hydrocarbon group.
 18. Thecoated nanostructure of claim 16, wherein R is an alkyl group, a cyclicalkyl group, an aryl group, or an alkylaryl group.
 19. The coatednanostructure of claim 18, wherein R is an isobutyl group, a methylgroup, a hexyl group, a cyclopentyl group, or a cyclohexyl group. 20.The coated nanostructure of claim 6, wherein the first coating isselected from the group consisting of:

where R is an alkyl group, a heteroatom, or an electron withdrawinggroup; and

where R is a halide.
 21. The coated nanostructure of claim 1, whereinthe first physical property comprises solubility and the secondelectrical property comprises nonconductivity.
 22. The coatednanostructure of claim 1, wherein the first optical property compriseslight emission at a first wavelength and the second optical propertycomprises light emission at a second wavelength.
 23. The coatednanostructure of claim 1, wherein the first coating is converted to thesecond coating upon application of heat.
 24. The coated nanostructure ofclaim 1, wherein the first coating is converted to the second coatingupon application of radiation.
 25. An array comprising a plurality ofdiscrete coated nanostructures of claim
 1. 26. The array of claim 25,wherein the member nanostructures are present at a density greater than1×10¹⁰/cm².
 27. The array of claim 25, wherein the member nanostructuresare present at a density greater than 1×10¹²/cm².
 28. The array of claim25, wherein the member nanostructures are coupled to a surface of asubstrate.
 29. The array of claim 28, wherein the substrate comprises asilicon wafer.
 30. The array of claim 28, wherein a first portion of amember nanostructure is coupled to the substrate, and wherein a secondportion of the member nanostructure is covered by the first coating orthe second coating.
 31. A device comprising a plurality of discretecoated nanostructures of claim
 1. 32. The device of claim 31, whereinthe device comprises a charge storage device.
 33. The device of claim31, wherein the device comprises a memory device.
 34. The device ofclaim 33, wherein the device comprises a flash memory device.
 35. Thedevice of claim 31, wherein the device comprises a photovoltaic device.36. A coated nanostructure-containing composition comprising: aplurality of nanostructures; and a coating separating each membernanostructure, wherein the coating comprises a plurality of moietiesthat bind to a surface of the individual member nanostructure; whereinthe coating is convertible to an insulating shell after deposition ofthe coating and binding of the moieties to the surface of the membernanostructure.
 37. The nanostructure-containing composition of claim 36,wherein the member nanostructures are coupled to a surface of a siliconsubstrate, wherein the silicon substrate further comprises a silaneligand coupled to a second moiety that binds to the membernanostructures.
 38. The nanostructure-containing composition of claim37, wherein the second moiety comprises one or more phosphonate ester,phosphonic acid, carboxylic acid, amine, phosphine, sulfonate,sulfinate, or thiol moieties.
 39. A plurality of discrete nanostructuresencompassed with rigid SiO₂ shells, wherein a diameter of a membernanostructure with its shell is less than about 6 nm, and wherein themember nanostructures are present at a density greater than 1×10¹²/cm².40. A method for post-deposition shell formation on a nanostructure, themethod comprising: providing one or more nanostructures having a ligandcomposition coating a surface of the nanostructure and thereby forming afirst coating on the nanostructure, which first coating is curable to arigid shell; and curing the first coating and generating the rigid shellon the surface of the nanostructure, thereby forming the shellpost-deposition.